I am trying to fit 2 subs under my seats because that is where the interior of my car is lacking bass the most. Under my seat is a 5'' (depth)X 11''-12'' (with) X 18" (length).

i am considering this sub Pioneer TS-SW2501S4 10-Inch Step Up Shallow S4 Subwoofer (mainly because of my all pioneer set up and it fits)
found here:
Amazon.com : Pioneer TS-SW2501S4 10-Inch Step Up Shallow S4 Subwoofer : Vehicle Subwoofers : Car Electronics

It is the only 10" sub i can find that i know will fit but i would have to do it in a free air config under my seat. So what i really want to ask is should i get this sub and do a free air config or do you think i should try to fine a 8" sub with a small enclosure that i may have to tinker with to make fit? Oh and it would be nice to know what i would loose (base,air movement to much/litttle) in a free air config with this sub or really any sub?