AR Ref 600 question's

I actually own a pair of Ref 600 II, with eidolon diamond's and cj act 2 preamp. I find the sound outstanding for imaging and power, but the ref's sound is a little on the dry side, compared with my previous cat amp's.
My question's basuically are two:
1)retubing with kt 88: a good idea? witch kind of tubes?
2) retubing 6922: mullard nos, amperex bb or telefunken nos?or other brand?
Thank you very much for help....
Alexander