[Chuck Schwark]

Other things to consider.
Bad speaker voice coil rubbing?
Easy sub. Just disconnect the VC wires ONLY and hook to a PM speaker to check.

Partially shorted audio output transformer?
Harder to check without a full sub.
D.C. resistnace measurements may show significant difference from
service bulletin. Check both Pri and sec windings.

Changed the 42 audio output tube?
Antoher easy swap and check.

Improper grid bias on the 42 output tube. BE SURE the (-) end of the dual 8/4 uf electrolytic cap DOES NOT connect to ground or chassis.
A common mistake if you don't watch the schematic carefully. The negative end connects to the jct of the powers upply transformer secondary center-tap and R30 and C28. See schematic.

Have a scope? It would be easy to see where distortion is starting and trace.

Chuck