damn....triple post there blaine...anyway, you dont HAVE to have a new HU to power the amp. it just like isaac said, the more power coming out of the HU the easier it is to amplify. So its really totaly up to you whether you think you should get a new HU or not
higher preout voltage has a lower noise floor (better SNR) and gives a cleaner signal at the amplifier input stage. It also yields a higher resolution signal, technically. "taller" sine wave means more room for subtle nuances in signal fluctuation (more detail)
A higher voltage means almost nothing if the build quality isn't there. This falls under the pretence that a volt is a volt but in fact this is not true. If all in all is true and the quality is there, clean signal and all then what Glass says is true but I have seen POS head units with high pre outs but it sounded like crap due to the distortion. Try sticking with a well known brand or just ask others what they think.... Polo.
Like this one. Eclipse 55060 Competition level Head Unit. This is regarded as one of the best head units ever made. DOES NOT SUPPLY POWER TO SPEAKERS, REQUIRES AMPLIFIERS. Copper Chassis, Eclipse Security Network, 8V Output. This was installed for one audio competition, and then replaced in its box. Paid 1000, selling for $500 or best offer. Its a true SQ CD/Receiver.