You cannot replace the stock 6922's with E188CC's in a CAT Ultimate. ECC88's (6DJ8's), E88CC's (6922's), or CCA's are OK. Do not use US 6DJ8's (like Sylvania or RCA, but not limited to these brands) or E188CC's or E288CC's. The manual clearly states which tubes can and cannot be used in the Ultimate. You will risk an expensive repair of your unit if you don't heed the warnings.
As far as which tubes I like in the CAT Ultimate, I like my 1960's Telefunkens, both for the positions that take 6DJ8/6922 and 12AX7. I have tried Amperex, Siemens, Mullard, Mazda, Tungsram and of course the stock Sovtek 6922's and the EI Yugo 12AX7's. Nothing for my money competes with the Telefunkens. I do agree with 42659 that the stock Sovtek tubes do not compete with any of the NOS brands mentioned above. However, the EI Yugo's are not bad even though the Telefunken's are better.