Tweaking my Cronus Magnum Tubes

I'm wanting to tube roll on my Cronus Magnum. I have the KT 120's and the standard small tubes that came with the Magnum.Also have the KT 90's. Should I update the KT 120's to Gold Lion 66,77 or 88's There's a quad of Gold Lion 66's on sale here at Audiogon.

W#hat are good small tubes to update and what small tube position are most important to update?

Thanks mates!
I bought a RCA nos 5814 for the gain stage from Brent. Nick at Rogue said the gain stage tube was the most important small tube. Should arrive tomorrow. I also biased the power tubes for the first time in awhile. 3 tubes were off by 5 to 15 points. This made a noticeable difference!
The RCA NOS 5814 is a great upgrade. Smart purchase ! I check my tube bias about once every two months...... rarely needs any adjustments. Enjoy your "upgraded" amp, and have fun listening to your music !