Don't shoot the messenger but..
For a pair of 15inch subs that cost under 250 total and will work well with 400 watts each and can be tuned low and have good SQ.
Pryamid PW1586X "Hyper Pro"
Ohm-4
VC-2.5" KSV
RMS-600
Sensitivity-89.5
FS-31.9
Qts-0.53
VAS-3.7960
Mounting Depth-6.14"
I know, I know it sounds crazy but some of their stuff does actually work quite well, mainly because they don't actually design or build it they just put their badge on it and call it their own, thats why you will see some stuff thats just garbage and other stuff that is pretty decent.