Why would 1 SS monoblock run hotter than other?

the hot one is probably clipping and needs to be serviced.

If these are not class A amps (sorry I'm not familiar with them) then they shouldn't be getting hot, and I would be worried.
If they are class A amps I'd be wondering about the cold one. :-)
The fact that you switched amps side to side means you've isolated the problem to the power amps.
Either way electrical problems can cause fires so I'd get an experienced person to check it out.

I don't know much about biasing but it can have a lot to do with the heating as well as the sound of the amp.

Perhaps the hot one's biasing has been calibrated at a substantially higher level than the other. If it is biased too high, then that can destroy parts of the amp also.

I'd check with Krell, their customer service is excellent. These are Class A amps and should run warm, but they do have cooling fans which sometimes were in need of replacement, a reason Krell switched to heatsinks in their next generation of amplifiers. Are the cooling fans working in both of them? If so, something's wrong with the bias in one of them, possibly, or worse.