What year f250..? There is more room behind the seat than you think. Most 10s should fit no problem. I did 4 TMA 10s behind my brothers f350 crew no problem. They are a fairly shallow mount sub, but I used a JL 10 w6 for test fitting, so they would fit as well and they have a large magnet.
6 15's fit nice in f250's //content.invisioncic.com/y282845/emoticons/laugh.gif.48439b2acf2cfca21620f01e7f77d1e4.gif. You should have enough room for 2 shallow 10's or 4 8's!
I have an 02 f250 crewcab and what i did is take the back seat out and moved the seat foward on the mounting brackets about 1.5-1.75 inches. pretty easy just had to drill 4 new holes where i wanted the seat to bolt to the bracket to move it foward. After that built a custom sealed box for 2 IDQ12's at 1.1ft each i could have made it bigger probably could have got 1.5 each if i neaded to. but was able to get about 6.5" of mounting depth. The IDQs sound fantastic but RE SE/x 12's or even FIQ 12's would work. Well worth the effort i think. Actually im waiting for the new IDQ15v3's to be released so i can try those. also bar grilles to protect the subs. and if your worried about leg room my buddys 6'3 and doesnt have a problem sitting in the back with it moved foward..
I heard a JL 13tw5 in a in a fiberglassed enclosure in the back of a 911 ovewr the weekend and it rocked!! I'm trying to descide on 2 of them or 3 Kenwood shallow subs in my car