Any recommendations for a subwoofer for my car and audio setup?

[RyanMacDougall]

I have 2012 accord coupe and in it I have a JL audio XD/400 amp running a pair of focal speakers model 165 in the rear and a pair of focal 165 cv polyglass in the front.. I'm not sure if the little tweeter speakers are hooked up to the amp or if the speakers in the front are running off of the headunit? Anyways I also have a alpine CDA-105 faceplate and a Boston Aucoustics gta1000m amp running a jl audio 12 inch W7 from 2003 and I bout it used (how can I tell if its in good condition?) anyways before that I had a 10 inch jl audio sealed w3v3-4 sub.. Problem with the w7 is it takes up my whole trunk and it makes my system sound distorted as its in a ported box with two vents on each side and it smells like ciggeretes lol. Now I want a subwoofer that won't take up as much room but I don't really want to downgrade or anything or overpower it at all.. Any help on this issue would be awesome thanks! Also if I turn it up the subwoofer starts to distort before the speakers why is this? Sorry for all of the questions I'm not too experienced in audio I just love my stereo and music!