Krell Amps. What are there major differance.

I currently have a Krell KAV 500/5, Lexicon MC1 and B&W metrix 3, 801's in the front 805's in the rear and HTM in the center. I am looking at upgrading to 7.1 and will most likly us a pair of B&W 805 Metrix 3's on the sides. I am pleased with the performance of the KAV 500 in this hometheater set up. I am now looking at adding 2 more channels. What amp? I could go with another KAV 500/2, but I was looking a maybe a little more power for the 801's. The options are now endless... KSA 100s or 200s (heat might be a problem as the amp area will be getting a little crowded), KST 100, KAV 250a etc. Any suggestions will be appreciated.
My multichannel system has Krell Ksa-250 for fronts and Kav-500 for center and rears. I have played around with my Kav in many configurations of my system. While it's a great little has it's limits. These limits (All Kav models have them) are reached much more quickly than all of the other Krells because they just don't have the power supply of their bigger brothers.

If you want to stay with krell, use the Kav's channels on the side speakers and pick up one of the newer (or older) non-Kav models for your fronts IMO.