Do I need new tubes?

That is an odd symptom. I'll second @jond 's  recommendation to put in the spare set of tubes... it'll take 10 to 20 hours for the new tubes to sound right. 

You can never have enough tubes!!

Hoping to piggyback on this discussion, maybe change the header to "DID I need new tubes?"

  Recently had my RM-9 Music Reference amp repaired, rather extensively, at George Meyer Audio in L.A. after losing the left channel. I'd been running 4 KT-120's, for several years. 

  I was told to replace the tubes, because they "had no gain."  I had swapped the tubes, and all other inputs, extensively trying to identify the possible cause of the left channel outage, and each tube glowed, produced output, and responded to volume control on the right channel.  

   Further, I was told not to use those 120's or I'd be bringing the amp back for repair again. He was not very instructive when I tried to ask him about all this.  

   So, did I need new tubes?  Which I've ordered by the way.  

Appreciate any input and feedback.  thanks.

Jeff

See if you can find a good audio shop close by with sophisticated tube tester, e.g., AVO or TV-7 and have them tested. Or swap all out from back ups, if you have them. Last resort, find a decent tube tester on ebay. I bought a B&K 606 for about $150. Basic good/bad on a scale, shorts and gas emission.  I own five tube amps and have over 250 vacuum tubes in collection so it was worth buying. 🤪 Just another expensive hobby.