No,your box will not fit under the sit, it is only 71/2inches high I am in the same boat as you I bought MTX Thunderform 5000 which fit and are made for are trucks,
I put a 2 channel mtx 250 amp under the front driver's seat, I just left my stock speakers alone.The sound is earth shaking, I forgot to tell you the box hold's to 8"s.
I like my music loud.If you want you could just put a four channel amp in 35 watts per channel should be fine but I don't no what kind of music you like.
Hope this helps. BYE

Expectations
 

I'll be getting my truck (99 F150 SC Lariat 4x4, Wedgewood Blue) next Friday. I have heard the radio(the in-dash CD player) and it is a little weak. I have two Rockford Fosgate BTR-83's 3way boxes . Will they fit under the rear seats? Will I have to buy new boxes to put the 8" subs in? Should I run an amp? I also have a pair of Polf 693ex 6x9's. Should I put these in 6x9 boxes and use them instead of the BTR's? I don't want to spend a whole lot of money now to get good clear sound (both crisp and boomy bass).

mike k-

how are you driving the amp, seeing as how the factory system does not have pre-outs? are you using a line-level converter, or do you have an aftermarket head unit?

thanks-

steve

Some amps have line level inputs. Simply rin speaker wires from radio to amp, then from amp to speakers. MY kenwood amp has this capability though I'm not utilizing it.

------------------
97 F-150 XLT S/C FlareSide 4.6L Auto 3.55LS Portofino Blue/Med Graphite, Gibson Cat-Back, K&N Element, AirBox Mods, SmittyBuilt Nerfs, DuraLiner, 285/75/16 BFG's.

they ain't gonna fit mtx thunder forms 5000 perfect glove fit don't put no garbage under your sit

Don't even think about running subs w/o an amp. you will be disapointed passive x-over would be required to kill the mids and highs. btw if you are going to drop the $ for a sub, by an amp even if it's a cheap one