As far as the amp, you could gut a pair of pc speakers (most guys upgrade thier speakers and have at least one set of factory pc speakers around. Most are powered by a dc9v adapter. So you can remove the ac/dc adapter and wire in a 9v to power the amp. I did this with a psp case mod and put in my own speakers/amp . You will be best off running your sub in mono on its own amp. and run the 20mm in stereo on there own amp.
this speaker is the better of the 2.
http://www.tb-speaker.com/detail/1230_04/w2-1625sa.htmYou might want to find 1 with a lil lower frequency response.
this is off the link posted above. I will highlight things to look for. high#'s will be in red, if something needs a low # it will be in green. Yellow is for matching to your amp. orange will be look for this in other products.
SURROUND MTL
SantopreneNOMINAL IMPEDANCE
8 W(this says "W" but i think its supposed to be ohms)
DCR IMPEDANCE
6.6 W (this says "W" but i think its supposed to be ohms)
SENSITIVITY 1W/1m
80 dB 80 is about as low as i would go here.
FREQUENCY RESPONSE
90-500 Hz 90 is a lil high for sub , but it will hit. you wont probably find a small sub that can produce sound in that traditional sub zone 30-100hz.
FREE AIR RESONANCE
90 Hz
RATED POWER INPUT 20 W
MAXIMUM POWER INPUT 40 W these power #'s seem to be ok