Most likely noisy small tubes, inherently noisy. Could be power tubes but usually this is not the case. FWIW the volume control usually appears in the circuit before the tube amplification and only attenuates the signal from the source. Next tubes you get should be 'low noise' tubes especially if you are buying NOS tubes which are costly.
You might just try shifting the small tubes - left channel to right channel, etc. Some times the issue is not the tubes so much as the pin contact in the sockets.
You might just try shifting the small tubes - left channel to right channel, etc. Some times the issue is not the tubes so much as the pin contact in the sockets.