Should I upgrade my Tubes

Assuming you’ve checked your bias, have more than one source and they all exhibit the same result into the Cronus: I’d replace the middle 12AU7 (preamp tube), first.      Generally speaking; tubes lose their juice more slowly (not a sudden event, in both channels).      If the loss was really that sudden; you may want to contact Rogue for guidance (570-992-9901).     Tube rolling/tasting can be a lot of fun.      If you desire the best sound: contact NOS purveyors (ie: Brent Jessee Recording, Jim at McShane Design, Kevin Deal/Upscale Audio, Andy Bouwman/Vintage Tube Serv), for advice given according to your listening preferences/tastes.    That can get expensive, as well as addictive.

If your amp can accept 6550's Svetlana , go with those,, I am running the Svet 6550 in my Defy7, That tube has a  solid reputation, and I concur lives up to the hype.
The new and old are IDENTICAL,, no such thing as NOS Svet are superior to the new, thats snakeoil, and some folks are getting 2x's the price. 

The 6550 will deliver all the same/or very close (= a superior balance bass to mids) bass as the 120’s, but also give you high res mids
What i am saying is
the 6550 (kt88 = same tube, but I perfer the Svet vs all other 88 labs) is perhaps the finest power tube ever made. 
and I perfer the Svet over all other 6550's