Should I upgrade my Tubes

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. 

Rodman Gave very sensible advice

GL KT77s are great tubes, although currently I'm using a pair (I have a quad of these) of unused Sovtek KT88s I've had for a while...somehow they sound great.

@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.