Sony Mobile ES XM 7557. Hard to beat in any aspect if you have space. Harder to find, but some out there. I'm running one in my daily. Not looking to sell, but if you can't find much else, I'll entertain offers.
Also, there are a bunch of 5/6 channel amplifiers in the DIYmobileaudio classifieds along the lines of JL HD's, etc.
If you have the cash, look into the Audio Systems amps being sold by a dealer here in the classifieds...I remember him having a nice 6 channel for sale.
i have an audison LRX DIRECT 6 HIGH END SQ amp for sale.. retailed back in the day for welll over a grand... you wont find better pure sq 6 channels anywhere