I've had 2 15 inch solo barics for the past 2 years(with a 1500D on each sub) and im use to songs hittin a certain way. I wanted something different so i purchased the nightshade 18(3000D pushing it) and the songs that hit hard in my 15z dont hit as hard in the 18. why is this and what songs should i play to get the full effect of this beast in my hatch??????

You should rebuild your box if you're not hitting certain notes.

^ Mhm, how about some currect box specs?

My guess is that your L7s were tuned to like 42 hz so they would play really loud in the 50 hz range. But the Nightshade is in a better box and plays the lower notes better and is not so peaky.

Give us your box size and tuning frequencies of each box.

Could be the settings on amp. Tell us both box sizes, tuning, and settings on amp(Low Pass Filter, SubSonic Filter).

Bradford Lofton your box is tuned at 30 HZ that's why certain sounds play differently on the 18 than your 15s. I'm sure the person that built you the box told you that. You have to start listening to what he tell you. Good day sir

Could also be a cancellation issue. Even the slightest change in application can yield dramatic results.