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

I used NOS octets of Sylvania 6550s for a few years, purchased from Upscale, until they ran out of stock.      Kevin Deal recommended the Winged ’C’, Svetlanas, as his number one choice for KT88/6550 types, and they were close enough in performance to satisfy.      When the Svetlana plant in St Petersburg burned (2012), production of that (very nice sounding and reliable) tube ceased forever.      I then found a supplier for octets of NOS GE 6550As, which were superior in every way.      My new favorite 6550/KT88 tube is the Gold Lion KT88 repop, from Russia.    A conversation with either Kevin Deal or Brent Jessee would be invaluable, far as the straight scoop on the sound and reliability of the current, "best", far as new production KT88/6550 types and what became of Svetlana’s version.      Some history, regarding the Svetlana plant and 6550 (Winged ’C’), here: https://valvetubeguitaramps.com/svetlana-winged-c-valves/      Lots of opinions, here (note the dates): https://db.audioasylum.com/cgi/search.mpl?searchtext=winged+c+6550&SelectForumtubes=tubes     Of course; everyone’s ears and systems vary.      A couple tube purveyor observations: https://www.thetubestore.com/winged-c-sed-6550c      and (bottom of page): https://www.tubesforamps.com/6550-tube-selection-guide

@rodman99999
"...My new favorite 6550/KT88 tube is the Gold Lion KT88 repop, from Russia."

During some of my own "KT88" replacement chats with a few colleagues and some tube resellers, the new Genelex Gold Lyon KT77 is mentioned quite often as well.

Ever try these and compare KT77 to others noted above?