Why would 1 SS monoblock run hotter than other?

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.

I would guess bias current is off in the cooler running amp and the amps won't sound the same under these conditions. You should contact Krell and see if they will tell you how to make this adjustment or someone to service it locally. There are normally trim pots to adjust but you need to know where to measure and what the current should be.

The KMA-100 operate in class A, meaning they're always pushing the same bias currrent throught the transistors whether at full power or idle. The continuous current flow generates steady temp heat which is ejected from those huge heat sinks on the sides. If the sinks are different temperatures with respect to adjacent ones or the other amp's, then you MIGHT have a problem with the input circuitry. I'm not sure about the Krell, but I would guess the IC is on a printed circuit board near the rear. One of your boards may have a damaged component (a capacitor or the DC coupling circuit). If this is the case, then the amp might black out sometime soon (curious to know how long you've noticed this).

First, take a meat thermometer and measure the temp of all sinks on both amps. Place the thermometer in the middle of the sink. They should be about 120F and not vary by more than 15%. Also, place your hand on the top of the amp by the power cord (where the input board should be). Both amps (if normal) will feel about the same temp, very warm. Then give Krell service a call and tell them the heat sink temps. They should be able to tell you right away what the problem is. Good luck and hope this helps.