Classic 50 410 - loud clipping distortion when notes played hard

[centuryhouse]

I bought a 2nd Classic 50 410 from a FB Market deal this weekend but due to pandemic stuff didn't get to try it out til I got home.

- on either channel there is a very loud, harsh clipping/distortion sound if I pluck the note hard or loudly strum; NOT a music distortion

- with reverb on it is much worse; I swapped a known good reverb tank and wires/plug and the same issue occurred (I also installed a new op-amp, no change)

- the issue becomes progressively worse as the NORMAL (clean channel) or POST (Lead channel) are turned up, regardless of how low the MASTER VOLUME section's VOLUME is set; the VOLUME can be set very low and the sound coming out can be very quiet - but if NORMAL or POST are high the noise is bad

- I swapped a good 12ax7 from another amp into each position one at a time and the problem persisted

- I swapped a good el84 from another amp into each position one at a time and the problem persisted

Any ideas? I have a meter but am not versed in using it so when people talk about testing whether this or that is 'open' or whatever I don't usually understand what they are asking me to check.