CAT Amp Power Tube Options...

Would like imput from any CAT amp owners on what they use for power tubes, I just retubed with SED Winged C 6550's the amp came stock with Svetlana "S" logo 6550's different Mfg. plant and much different sound.

The SED's are full and warm nice body but not as Fast and open on top as the "S" logo I am throwing around the thought of Cryoing the SED's to hopefully give me more extension and Air? (I have about 100hrs on the new tubes now don't think they will change much more) it is musical but miss the speed and openness of the "s' logo's but do like the body...

My question is what are owners using, Valve Art 6550 or anyone using KT-88's is there a resistor change to use 88's and what would you Bias at with them and most of all is this safe?

Thanks for your thoughts and options.

Tom