eek.. i don't think that would be a fun idea. If you wire both the door speakers and the subs in parallel, you're gonna get yourself a fried HU. If you wire them in series, you'll have 4 underpowered speakers, that won't sound too fun. I say have your brother buy an amp, so you can use it also.
Also, you'd need a xover for the subs. Unless it's a full range box. But either way, you've got yourself a lot of problems if you tried to push all speakers in the car with just the HU.
