For now, get the AQs/SQs. But only if you plan to double the power, because a "smaller" or less powerful essentially type of woofer would get "louder" on only 1.1lW each. So, to combat this, plan to build two enclosures. One on the larger side for the one amplifier set-up for now, to help with the lower power making things more efficient essentially increasing how "loud" they'll be but they might grow peaky the larger you go; then when you get the other amplifier and have 2.2kW on each woofer build another enclosure which could be smaller in size.
2.2kW on each HDC4 will be great in the future, IMO 2.2kW on each 3512 is getting towards the higher side of power handling (though they surely can take more if you know what you are doing, BUT, they start losing efficiency per se while the HDC4s will excel on the power and more //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif, without getting near or out of the efficiency area). Honestly though, a 2.2kW amplifier on each AQ/SQ HDC3 (C-copper) will be a great choice all around for now on only 1.1kW on each driver, and 2.2kW in the future. HDC4s can take, and need lots 'o powah, if you might be putting say 2.5kW and more on each driver eventually, an investment in the HDC4s would be a great idea for drivers you won't have to worry about getting stinky
with lots of headroom for power, so you'll be in the power area to where they'll be efficient but could take much more; ie they'll get loud, but could get louder with more power, so you'll get them moving and making tons of sound, but then you will have headroom and lesser chance of blowing them. But you know, set everything properly, if you start smelling coils, turn 'er down et cetera. If you have, or know of an oscilloscope, then fantastic.
But, are those your ONLY choices you want now? Because we could all recommend plenty of other drivers and whatnot for less, that'll kick just as much arse and the drivers you have chosen so far. But yeah, I recommend a different pair or single amplifier if doing the HDC4s eventually (not that they will not "move" on only 1.1kW a piece, and they'll do awesome on 2.2kW a piece, but in the end, I think you might be better off as a whole with the HDC3s for now and later. But still plan on two enclosures, not that it'd be as necessary as going from 1.1kW per HDC4 to 2.2kW to each. Really, you could probably get away with just the proper enclosure for the HDC3s for both the amounts of power you plan to run.
Oh, also, are you dead-set on running a pair of 12" drivers? Doing a killer (killer on 2.2kW that is) single 15" driver would get you right near where the pair of 12" drivers will, and you won't need the power increase all too much. Or say, if you can fit 5-7 cubes net, a single 18" driver on the single AQ/SQ 2200 amplifier will be pretty nuts inside an XB. Then you won't need more battery/alternator, nor another amplifier.
Seeing as how with a proper design and testing, I bet you could get close to 150dB with a tad over 2kW and a decent amount of cone area. I say this because my pair of 12" woofers on 2kW for the pair in my trunk does over a 150dB on the windshield, as well as a single 15" (as close as I could make it) into an almost direct drop in build, is only .4dB less. I am about to quickly throw together a quick and easy single 18" enclosure for the trunk and run the same power, we'll see what happens there on the same power and increase in cone area.
Like how I could build you a nasty driver that'll perform incredibly on that power, as well as tons of others on here could throw together a woofer for you depending on what exactly your goals are. And remember, 2.2kW is a lot of power if done correctly, plenty of us can do over 150dB with ease on or near 2kW.
