Have you tried contacting Krell. They should be able to help you.
There is no easy means to convert the frequency of your line power. Changing voltage is easy (step-up or step-down transformer), but to change frequency you need to literally "regenerate" the power. Fortunately, that's exactly what the PS Audio products like the P300 do. (And they can be intrenally switched to run off 220v).
However, I've never heard of an electronics product like a processor that wouldn't run fine on either 50 or 60 Hz. I just moved back from England and ran all my AV gear on 50 cycle power with no trouble. I suspect if you contact Krell, they might tell you to ignore the error message and press on. Does the unit seem to function OK, other than the error message?
The Krell HTS was a "short lived" product. It didn't have analog throughput, and required use of the Krell processor. Since this limitation effected the performance of SACD and high end redbook players dramatically, Krell quickly changed the product (HTS-2) and added a new Preamp (analog throughput) feature which provides for much better handling of 2 channel audio.
The HTS-2 will cost a bit more than the original but it is simply a better long term value if usability and system upgradability are concerned.