Phoenix Gold are pretty good amps. The one you found on ebay is rated at 75w X 4 channels which should be pretty loud. I really wouldn't pay much attention to the 800w claim on a Kenwood or Phoenix Gold amp. That refers to the peak power of the amp and not the continuous output. The RMS (75w X 4 for the Phoenix Gold) is a better indicator of the true power of the amp. It doesn't say whether it's 2 ohm stable. If it is, you can basically double the RMS rating and that's the output you'll get at 2 ohms.
The diamond audio amp I mentioned is 2 ohm stable and you'll get around 100w x 4 if you wire the speakers in parallel (2 ohm wiring). Either amp should be as good or better than what you're getting from the Kenwood. Kenwood is a good brand, but they tend to put their peak power ratings on their amps to make them appear more powerful than they really are. You can go to www.crutchfield.com
and do some research on RMS vs. peak power and find some more information on wiring your speakers correctly to maximize your amp output.