From talking to an experienced tube guru he advised me that you could mix power tubes from different manufacturers as long as you have the same tube set-up in both monoblocks. They should be identical on each side. Say One KR KT-88 and One GE 6550 tube in each monoblock.
If I was in the OP's situation I would just sell off the KR KT-88 tubes and use the matched quads or pairs available. My preference is matching quads or matched pairs on each monoblock if matched quads weren't a possibility.
If I was in the OP's situation I would just sell off the KR KT-88 tubes and use the matched quads or pairs available. My preference is matching quads or matched pairs on each monoblock if matched quads weren't a possibility.