Impious

No wonder you were peaking so high...your enclosure volume looks much better suited to a single hx380d than a pair.....

The "loudness" and "source level" should be set where ever you would normally have them set. I'd recommend the Loudness be off and the SLA where ever works for you, just don't go crazy with it. While it's not technically wrong to use the HPF on both the amp & HU, it's not necessary. Just use one and use which ever is easiest for you. M5 was saying to turn them both off, and then gradually raise the HPF (which ever one you choose to use) until the response/sound is acceptable. Just be VERY careful with the volume as you can mechanically damage the drivers while the highpass is off or set to a low frequency.

You had those 2 Hertz in a single 4.2cuft enclosure?

Right, because being 1ohm stable is what determines how good an amp is......

And Alpine designs their own products rather than going to a Korean build house, selecting a board and case like it's a fast food restaurant and then selling it on the open market. It's much cheaper to buy a board from a buildhouse, who sells that same board to anyone with enough money to meet minimum order quantities, than it is to design, produce and market your own product.

So you tried a different sub and still had issues with output? Measure the voltage output from the amp and see what it reads.

No matter how it compares to your other cars, the door panel is not going to be a good mounting surface for a speaker. And you need fix those air leaks. Kind of silly to spend money upgrading your stereo to leave a ton of performance on the table by fouling up the install.

So the current front stage is (supposed to be) 3-way with the 5.25's and tweeters on the dash and 6.5" midbass in the doors ?

According to WinISD in that enclosure volume (4.2cuft) with the same power the 18" would peak a little higher in frequency and have up to ~2db more output above tuning, but it rolls off a little sooner so the 15" would have maybe 1db more output over a very narrow bandwidth right where the 18" starts to roll off around tuning. But the 18" would have about half the group delay around tuning, so the transient response might be a little better. And that amp is perfectly fine for those subs.

They are exactly the same as a normal speaker and the exact same rules apply. The physics don't change.

Go with the 1kw. Don't rely on birthsheets, they likely aren't very accurate. And a couple hundred watts in either direction at that power level isn't going to really matter.

How many bands is your EQ and is it graphic or parametric? And what are the center frequencies? Having both xovers at ~120hz plus a -12db EQ adjustment at 160hz seems excessive to me.

The highpass crossover frequency probably isn't going to help with the issue you are having, unless you are completely overdriving the speaker with the current crossover setting. Where do you have it set now? Do you have any issues with the midbass sounding stressed or strained?

Looks like spring might finally be close. Next 7 days are supposed to be close to or over 60. So sick of this cold weather bullshit.

Also would be helpful to know what type of EQ you are using, assuming you are using one. But like the others have mentioned, fix the xover first.

One thing to remember.....a measurement is only good at the specific location you are measuring. You might take one set of measurements, dial everything in....move the mic 3" to the left/right/up/down and the measurement looks like shit again. And we don't "hear" from a single point in space like what you are measuring. Your ears are on the side of your head about 8" apart, and your head isn't always in the same location. Moral of the story.....you need to do spacial averaging if you want the measurement/tuning process to actually be useful and not just an exercise in futility. If your measurement software can't do that, getting one that can would be a good idea. ARTA I believe can do spatial averaging and is a free download. Second it would be helpful if your mic was calibrated or if you had a calibration file so you know if what you are measuring is actual system response or mic response. You might be EQ'ing something that is an artifact of the measurement device and not the system. Last, resolution to 1/96th is completely unnecessary. We aren't very good at identifying high Q (narrow bandwidth) anomalies. And like Sean mentioned, very helpful to know what type of measurement you are getting....if it's gated or not, and if it's gated what the window is.

Chances are good you'd be able to find both of those locally at some shops to demo. Those are both pretty popular brands. Not an ideal situation but better to hear them for yourself. There isn't enough information available to give a highly informed recommendation. I like that Alpine uses a split-gap motor design to linearize Bl over excursion and a shorting ring inside the motor.....off of no other information I would probably lean the way of the Alpine's but again your ears may tell you something different.

Is this for SPL competition, or a daily driver/showoff type system?

Yeah, forgot the amp was in the title. I would try grounding directly to the battery first and see if that solves your problem.

What is the amplifier? Does it only happen when you have the volume up high? If your battery is in the trunk why didn't you just connect the ground directly to the battery?

Sundown was about 20 years late with their surround design, those have came and went as has most of the products on the car audio market. I fail to see anything new, different or even reasonably necessary with the PS 8" driver. He says it was designed to work in small enclosures for people with tight spaces, which to me means small sealed enclosures. But the driver is going to perform horrendously in a sealed enclosure, so there's no use for it there. It's going to need to be ported, and small low'ishly tuned ported enclosure aren't a great idea if space savings is your goal due to the long port length. Sure someone could go with a PR, but by the time you account for the space taken up by the PR and the cost of the PR you either aren't going to end up saving much space, or in approximately the same amount of gross space you could have went with a larger driver that is going to outperform it. So what exactly is new, or even really useful, about this driver that would cause it to make sense in it's intended application or really any application? If it actually excelled in small sealed enclosures I could understand the intent....but it doesn't.

Could be several things. Could be those songs are heavily compressed and have little dynamic range, so as you increase the volume you are clipping the amps (inputs, not just outputs) or running into power compression issues on the drivers.

Using a DMM is not the best way to set the gain. Neither is using an o-scope. Nor the DD-1. Simplest and best solution is to set it by ear.

I don't think it's possible to describe beaming in subjective "sounds like" terms. It's also going to "sound" different depending on the severity of the beaming (i.e. how far off-axis you are from the speaker) and the frequency range over which it's occurring (midrange, treble, etc). Every speaker will begin beaming at some point, from subwoofers to tweeters. Beaming is basically a reduction in the amplitude of high frequency off-axis response of the speaker compared to the on-axis frequency response. If you were walking around a single speaker playing full range you would notice the high frequencies start to taper off as you walked from on-axis to off-axis. The higher frequencies wouldn't be as loud off-axis compared to on-axis. Where the driver begins to beam depends on the diameter of the speaker. A larger speaker will begin to beam lower in frequency than a smaller speaker. Beaming can create several potential problems....undesirable frequency response at the listening position from the direct sound from the speaker due to the "dip" or "hole" created by the beaming, uneven frequency response between the left-side and right-side speakers since you generally don't sit at the same degrees off-axis from both speakers, and larger deviations in direct vs reflected sound at the listening position. All of this cause issues with frequency response and imaging causing things to simply sound wrong. You might not be able to point a finger at it and say "Ah ha, that sounds like beaming!". But if the problem is in the midrange (for example) then the midrange frequencies might sound wrong or off compared to the rest of the bandwidth and the imaging will be skewed or scattered, for example. FWIW, I really like Andy Wehmeyer's philosophy behind designing a car audio system. He didn't invent the idea, but he's one of the bigger proponents and has spent a significant amount of time explaining it from an acoustics stand point. Pretty simple really. Cross all of the drivers within their piston range to avoid beaming, wave guide the tweeter (since you can't avoid beaming) to create a smooth on- to off-axis transistion, and don't mount the driver's right next to reflective surfaces (i.e. avoid A-pillar or top of the dash installations). Certainly it's possible to break the rules and create a system that sounds subjectively good without taking all of this advice....but it provides a good foundation to start from if you keep these things in mind.

First, the subwoofers don't "need" any particular amount of power. It doesn't matter if the amplifier provides 500w per sub or 400w per sub, you won't hear a difference. As far as the amps....you again won't hear a difference between them. Go with which ever is cheaper, which I'm guessing will be the PPI. The PPI's are solid amps, do rated power and there are a dozen different brands that use those same boards so they are relatively proven.