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!