KT120 replacement options in a Cronus Magnum II

First off replace all 4 power tubes together with 4 new tubes. Then bias according to manufacturer recommendations. If the amp is old and the tubes are that tired , I’d consider replacing the small signal tubes as well. Be careful not to damage the amp with a faulty tube, or powering it up without a speaker load. As far as your question , I’d consider the new production Gold Lion KT 88’s, great tube. They will be slightly less power and slightly less bass, but would be fine. I’ve also witnessed my friend going to Gold Lion KT 77’s in his M 180 mono blocks. Rogue ships a lot of its gear with JJ small tubes, and there is a ton of improvement to be had. This is all based on the assumption that your amp is healthy and has no other issues. Regards , Mike B. 

Check out Viva Tubes. I've never been steered wrong by them. 

Agree, replace all 4 tubes and re-bias when you're changing types. Also agree the Gold Lion KT88's are great tubes. I've rolled them into my Rogue Apollo Darks and I think there's a good argument to be made they're better than the stock KT120. The GL 88's have a sweeter midrange and an overall more refined sound. 

In the past I compared EH KT90 versus KT120 - the KT90 are brighter and leaner. I greatly preferred the KT120, though systems which lean a bit dark (need a lift in air and sparkle) could conceivably benefit from the KT90.

In other amps (VAC amps - not Rogue) I've compared EH KT88 versus GL KT88. Here the EH KT88 are a bit more rock 'n roll; the GL are a bit more refined and smooth. I put the Gold Lions back in, but I really like the EH 88 and keep them around for a change of flavor/pace some day. I don't think you can go wrong with any of the modern Russian KT88, and the Gold Lions are an easy general recommendation. 

That sucks about the KT120 shortage - a lot of tube amp makers have been using that as a "stock" tube in their amps for a while now, and I wonder if the tooling (dating back to 2009 / 10?) is getting worn down :(