I'll be running a Hertz MLK 165 component set and a 10" sub off of a JL HD900/5 so i'm ok up front and basically did what you said and picked up a used amp (US Acoustics 2150) from a fellow forum member to run the Hertz HCK's in the back.Assuming that you have a strong front stage (100+ watts/per side) already, then it isn't a bad idea to amp the rear speakers. If you're looking for something used, there are plenty of nice amps in the classifieds section.
I agree with this. Note, though - I have a Q4.150 and they run HOT. Not sure about the other two amps there; haven't used them.I would get a 4-channel. Use it bridged for now or setup it stereo and go active.
MB Quart Q4.150 (Q4150) Q-Series 4-Channel Car Amplifier
WoofersEtc.com - KAR 400.4 - ARC Audio 4 Channel 400 Watt Amplifier
Bazooka MGA4150 4-channel car amplifier — 100 watts RMS x 4 at Crutchfield.com