Hey, I made an account just to reply!! When I got a sub & amp fitted in my Adam it was £100 fitting + £30 for a part that was needed for the cables behind the screen or something. he got it done within 2 hours so I guess it must of been a easy install!!
I highley recomend the under seat sub/built in amp! There a little 8" sub and work great, i had one in my boxster, and then put one in the spare wheel well of my jag and it makes so much differance, its not boy racer standard and wont get the windows and mirrors rattling, but certainly make the bass sound 10000 times better than standard without taking up the boot space.
Oh they also come with all the wiring, i think the bf has two sat in the garage that hes not using using! The clarion and the ripspeed ones are the best (ripspeed is a rebadged clarion unit)
Oh you can connect it to the sub pre out on the back of the stereo, or if you havnt got and pre outs (i dont know about the adam) then you connect it to the rear speaker cables and it senses the bass from that and runs that way. Hope that helps?