It’s probably not the tubes (those NOS 5Y3 tubes are nice!) or the wall voltage. The Two Stoke should sound great running flat out. My guess is parasitic oscillation. Does the quality of the undesirable tone change when you change the “boost” switch setting? Try pushing the leads (wiring) around a bit with a chop stick and see if you can affect any change. Sometime even a very small change in the lead dress will cure the problem. Also, confirm that your grounds are good and solid, some times a weak solder joint will cause a problem. Start there and see what happens.
Please send the NOS tung-sol 5881 and NOS rectifier tubes to me at once so I can do extensive testing and confirm it’s not the tubes, I should be able to complete the tests in a couple of years… thanks in advance.