I believe the Ma Audio Hard Kore 12's I had were 38mm Xmax peak to peak. I wouldnt run a pair of subs bigger than 12s good luck fitting anything bigger facing forward. Im sure it can be done but I dont like the response of anything bigger than 12's either. Next set up like I said before will be 2 soundsplinter or DD 8's in the 6x9 location and walled off in the trunk. Maybe 10's will fit in the rear deck....