Bumpin' Goalie

Aero ports should be mentioned. They save a significant amount of space and are easy as pie to use. Could allow for a ported 12. PSP Aero Ports, check into them. They're sold at PartExpress and MadiSound for sure.

This is the biggest mistake people make. You think because the RMS rating is higher, it needs more power? I still saw the "Give it RMS" a few posts ago, lol....... Are you an efficiency guy? And yes, RMS is not THE determining factor. Often stated by companies to give the consumer an idea of the coil's thermal ability. Obviously there are a ridiculous amount of variables that affect, well, everything. OP seems to want to compete while retaining some "quality" (to me this starts with a flat response curve), thus compromise will be key here, especially when designing the enclosure.

Enjoy!

Yes you will be very happy with 2 15" lvl 4's. Though you may also want to consider 1 18" Level 4xl or even an 18 DD9518 if you want a single sub setup. And i still love the idea of the 21" IA DP for the "wow factor" no matter which you choose, youll be stoked. This may be repetitive, but do you have the electrical to support 2k rms? Most stock electrical systems would not appreciate that current draw lol.

With 2k rms and a proper charging system I'd go either 2 Level 4's (whatever you want to fit volume wise) I'm running a single 12" Level 4 off of 1200rms and its treating me well. The Fi BTL is a nice woof woofer, but i'd go DD over it if the budget is reasonable. If loudness is the main desire and space can be sacrificed, get the biggest woofer(s) you can fit properly to maximize surface area. Also, You may want to look into an IA DP 21" (it'll blow your mind )