Should I upgrade my Tubes

I had the same amp. Found that KT90s sounded better than KT120s, especially on voice.

Btw, not to go in another direction on this thread, but —
Replacing the fuses had more of an effect in this amp than anywhere else in my systems. My guess is because the fuses are DIRECTLY in the audio signal path in this amp. Just trying to be helpful. 

Call me picky, but I have some great 6550s Tung sol (GEs) they make the MC60s just sound wonderful, NOS 1964s I think.. A set of 8 NOS would be 2k easy more like 3-4k.  

KT88s sound NOTHING like a 6550, 6550 is US made KT88 is across the pond stuff.  The old Golden lyon is another great valve, but to be honest higher failure rates, than I'm use to.. (the new stuff anyways).

A lot of the newer KT88 stock is real close in sonics, to me. But a 
1957-68 or so GE 6550. Just one step beyond better...

PSVANE is a real good valve, you might want to look into that one to..
Very positive reviews from some pretty snobbish valve guys...

Regards

I'm concerned that you report a sudden loss of power.  For me this suggests something else that just a bunch of tubes gradually wearing out.  After checking bias you need to check the tubes individually, starting, as already indicated, with the one nearest the front, that is shared by both channels.

Did you re-bias the KT120 tubes?  In my Cronus Magnum 1, the factory supplied KT120 tubes had 3 years on them and I played the amp every single day.  They still sounded great.  Check the 4 fuses where you bias the tubes.  1 or more may be blown. If that isn’t the case, check the preamp tube, it may be dead.