I'm not sure what do you means by sub with low Db and still wants SQ/SPL (Btw there is no SQL) If you mean by sub which able to play low, high SPL and accurate my vote goes to
1) 2 X 15" Fi Q
https://ssl.perfora.net/www.ficaraudio.com/sess/utn;jsessionid=154574d1bd4d406/shopdata/0020_Q/product_overview.shopscript
2) 2 X Sound Splinter RL-p 15
http://www.soundsplinter.com/rlp15_premium_series_15_inch_DIY_subwoofer_information.html
3) 2 X 15" Kicker CVX
http://www.kicker.com/06/kicker_home.html
4) 2 X March 5 Maw15
http://mach5audio.com/index-2.html
5) 2 X 15" dayton titanic
http://www.partsexpress.com/pe/pshowdetl.cfm?&Partnumber=295-420
My vote highly goes to Fi Q for performance vs price
as for amp, you can go for
Hifonics Brutus BXi 2006D
http://www.maxxsonics.com/hifonics/amps_brutus.html
or Sundown Audio SAZ1500D
http://www.sundownaudio.com/