Try getting a speaker that has a slightly darker presentation. Bright sounding speakers would not be a good match. Also stay away from tube friendly speakers like Proac. The Krell will work well with Planar/Ribbon type speakers.
I know that JM Lab work very good with Krell, but you not only have to look at matching speakers but also match the rest of your gear, as far as speakers go if you can find JM Lab Electra 915.1 or 920.1 second hand, that would be a great match for krell.