I have not tried those tubes in that amp but in a Consonance. The KT88's give a fuller sound with a midbass emphasis. The EH 6550's (non cryoed) are more evenly balanced with more detail on the top end.
I got my cryoset KT88EH matched octet today. I won't be able to try them out until mid-January, because I'll be out of town. I've heard a lot a good things about the Electro-Harmonix tubes, so I'm looking forward to the swap out.