Two quick questions
 

Has anyone put an amp under the front seats in an ext cab? and will 6 1/2" speakers fit in the front and rear doors with adaptors?

planning to get Sarah a system for christmas and i need to know.

yes
and yes

just make sure its vented and wont overheat there
and
my type x's came with adapters for the 6.5 hole

i know they rear doors on mine are 5x7's and i think 6 1/2's in the front doors and as long as it gets enough air flow it will be fine

Yeah ditto to what everyone has said. If your worried about the amp there are cooling fans out there that can fix your problem. But you should be alright.

i have an amp under both of my front seats. i did tho have to take the BFH to my drivers seat because it scrapped ever so slightly on the back side of my seat bottom. i have 6 1/2s in my fronts. if you need brackets made up let me know i can make up a set for you.

i think Crutchfield sends free adaptors if u ask, ill make sure before i order.

i know about airflow, i been doin audio for 8 yrs but thanx for the advice. i wont be afraid to use a BFH, what she dont know wont hurt her :-D

How much bass response will i loose from stepping down from 6 1/2" comp. to 5 1/4" comps?