do they all have amps under the driver seat? 

i've got the columbus stereo, but the speakers (i've put pioneer ones in) are terrible. if i can replace an amp with an aftermarket one, that opens up more speaker options. 

i had a search but could only 1 one post which had one reply about fitting an under seat sub, but i want to do more than that

No the amp was an option. 

 

If you have these Sound System speakers in the front door, the you'll have the extra amplifier.