Help -- my system is too bright

I think the "problem" is your the Monitor Audio's ultrasonic ringing. I've never heard the 60s but I've had the Studio 50s for three years and I (still) own a pair of Studio 20s. The MA tweeter has a very prominent ultrasonic peak that some people find fatiguing. It's possible that the Krell is exposing this peak more clearly than your previous amp. If you haven't already done so you should remove the mesh tweeter guard. You can carefully pry it off. The speaker is also sensitive to toe-in angle. Try listening slightly off axis.

The ringing is somewhat inherent in the design of their tweeter. I remember seeing evidence of it in an old Stereophile review of the Studio 20s. They showed a waterfall graph with a severe resonant mode in the 22KHz-25KHz area. It was reported as being the first breakup mode of the metal dome tweeter. When I had the 50s I had them in a system with either Rowland or Pass electronics and Discovery and Sumiko OCOS cables. With the toe-in adjusted so that I could still see a substantial portion of the inner speaker panel and a listening height several inches below the tweeter level I never noticed the resonance "problem". I've read that it's relatively easy to design a crossover that traps out the offending frequencies, but I believe Monitor Audio purposely has not taken this approach because they think it does more harm than good.

Assuming that the 60s are even better than the 50s, you've got a very fine speaker. I hope you can work out this issue.