Sorry, I missed your response. To be more accurate, the turn off noise was more of a major fart than an oscillation. Whatever the final analysis is, putting the correct value caps in the circuit solved the problem. Now that I've read a lot more about this circuit, I probably should have gone with a little more capacitance for the first cap, but it's working well, so I'll just leave it. The amp is not back together yet as I still have to replace the power cord, remove the death cap, replace the power switch, and possibly replace the treble control. It's sweet not being in a hurry and being able to do the work when the mood hits me.
Ever since getting back into repairs after a 30-year hiatus, I've really grown fond of fixing tube-type guitar amps, particularly Sunn and Fender. At the moment, I only have one candidate for a repair. Maybe I'll look around on Craigslist and pawn shops.