Should I upgrade my Tubes

http://www.audiotubes.com/12au7.htm

An easy, more noticeable (generally) and less expensive way to enter the tube-tasting whirlpool, than swapping power tubes: a change of the preamp tube.      There are a number of nicer 12AU7/ECC82 tubes, that will change the Cronus’ presentation.        Mention your tastes to any of the tube mongers I’ve listed.      They can easily guide you to sonic bliss (kinda flowery, but- yeah).      Some opinions, here: http://hktubeaudio.homestead.com/files/best_of_the_best_12au7.htm      One nice thing; many of the better NOS, nine-pins are rated for 10K hours.       That tube will need to be tested/certified for low noise.      Any good seller will know that and provide such.

You have one preamp tube (the middle 12AU7).      The rest of the nine-pin miniatures, comprise the front end of the Rogue’s power amp section.      If it’s sounding good; no (no worries).      Those will typically last much longer than the output tubes.       KUDOS, on finding the issue.

@decooney - Never did try anything with less potential power than the KT88/6550, being an SPL lover and trying to squeeze as much as possible, with the lowest distortion, from my modded SLM-100s.      A relatively small, highly treated room with 93dB/2.83V speakers (w/active biamping and SS, below 80Hz, 60dB/oct) helps, but.....                                            The best presentation I’ve had, was provided by the GEs.     Glad I never owned an octet of the original Genelex or Tung-Sols.      I’d have probably been ruined for life.      Would have been nice, had the GL KT77 been available when I was modding/updating ST-70s.

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

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.