You need to check bias on all tubes. input/driver tubes can also go bad, but my suspicion that bad power tube could "take away" other components so bias checking is essential.
Chasing distortion
I have an Audio Hungary Qualiton A50i. Recently, one of the power tubes (KT120) red-plated and I purchased a replacement. When installed, I heard distortion, along with music, coming out of both channels. The distortion sounds similar to a blown speaker.
Thinking maybe the replacement tube was no good, I swapped the entire quad of power tubes with a set of KT150s. After installing them I found that the sound was significantly lower, but I could still hear the distortion.
What should I try next? Start swapping the input tubes (2) or the driver tubes (4)?
Thanks!