Tube recommendations for my Rogue Audio Cronus ...

It never hurts to be proactive. If you have 3 to 4 years on your tubes , I would say replace them.

I have been extremely happy with my GL KT-77's. I'm just thinking ahead, as we all know (or am I wrong ?) that power tubes should be replaced every four years or so. Should I be proactive and replace them in a few months, or should I wait for a tube to fail ?

I agree that the GL KT-77 sounds great. If you liked the Svetlana EL-34 you might also want to try the Tesla E34L tube.

In my VTL ST-80 I have never found better sounding tubes than the Genelex Gold Lion KT-77's. Unless you need more power I would get another set of the Gold Lion's. Also what makes you think you need new ones. Find a tube tester and check them out. Unless you are just looking to change the sound.
Alan

Contact Andy at Vintage Tube Services. He can offer some great recommendations for your equipment. I use him exclusively with great results.