Do the Havocs (namely 18"s) prefer to be ported or sealed. Or are they at home in either enclosure?

They'll work just fine in both. They have the linear stroke for good low frequency performance in a sealed enclosure and the electrical damping to achieve good output in a ported enclosure around resonance.

A pair will be going into the Tahoe in my sig. I'm wanting as "tight" bass as possible from these, and I'm leaning towards large sealed enclosures... Somewhere in the neighborhood of 4-5 cubes a piece. Any issues with this setup?

Nope, that should work just fine. I think you've drawn the correlation well: the sonic characteristics (including the "tightness" you mentioned) are largely controlled by the enclosure. You're going down the right path.

Now... Aiming. These are going in a Tahoe. What's the best direction to aim them? For looks in general, I would like to aim them at the rear doors, either slightly angled up, or close to a 45 degree angle. I don't see any issue with that configuration... The next part is where I think I'll have to modify: I wanted to put one in each "corner" and angle them towards each other maybe 30 degrees.

What would the recommendation be for this?

With a 10cuft enclosure I might be more worried about fitting it in the Tahoe cosmetically than direction of firing, even though I am sure the SPL crowd will hop up and say rear firing...

I still haven't managed to sit down and measure it out. I'm looking for some method of getting a bunch of 6" X 6" X 6" cubes to stack up to get an idea of how much space it's going to take up. I had the bright idea of using foam blocks for floral arrangements... Then I saw the price tag on them!

I'd just go to Home Depot and get some big ass appliance boxes to do a mock up with.

Now... Aiming. These are going in a Tahoe. What's the best direction to aim them? For looks in general, I would like to aim them at the rear doors, either slightly angled up, or close to a 45 degree angle. I don't see any issue with that configuration... The next part is where I think I'll have to modify: I wanted to put one in each "corner" and angle them towards each other maybe 30 degrees.

What would the recommendation be for this?

Aiming them back gives more "thump" to me. If you aim them up, it tends to smooth out the notes a bit, a little more musical IMO. I have an RE SX 15 in a 5.5 cu ft box, 33hz, port and sub facing up and it sounds nice. I have done several tests with the woofer and different box designs. I get more SPL with the woofer up and port back, but seems to have a better overall sound with the port and sub facing up, just not quite as much spl.