Did I lose a tube?

I'm getting thin sound out of one channel, but only thru the speakers. Is this normally indicative of a weak/bad tube? I have had this preamp (second owner) for a little over a year, maybe 18 months and have not replaced amy tubes. Here's my set up:
Technics SL-1400MKII, Ortofon 2M Red, and Cambridge 640P feeds the input of a CIAudio headphone amp.
The output of the CIAudio is fed to the phono input of the Rogue 66 Magnum tube pre-amp. That goes into the Anthem MCA-20 and out to the Vandersteens.
All interconnects (AQ Diamondbacks) are tight and seated properly.

The sound coming out of the CIAudio into the Sennheisers sounds clean, strong, very nice, equal in both ears.

The sound coming out of the Vandersteens has a "thinner" right channel.

The Rogue was my first foray into tubes. Does this sound like a bad tube?
Thanks in advance.
Yes, it DOES! Swap all the tubes(channel to channel) and see if the, "thinness" changes channels too. If so- swap one pair at a time, and that will indicate which one is weak. It'll require a lot of turning off and on, but will save you the cost of a tube tester, or a trip to one.
Thanks Rodman!
Hopefully, It'll be that simple. Happy listening!