i think i got what you were saying, i just wanted to make sure you were running the OUTPUT VOLUME pot at 2-4 and not 1.
i see that you changed the tubes, but did you pull them one at a time, let them warm up and then see if removing of a certain tube fixed the problem? The problem might not be the actual tube, but it might have to do with the part of the circuit connected to a certain preamp tube.
i would start and remove V1, closest to the input jack, try and play through the amp..and work your way down the line. V1 and V2 are going to be the primary preamp tubes. V3 is half connected to the drive channel and have connected to the FX return. V4 is the secondary section of the rhythm channel, V5 being for reverb, and v6 being the phase inverter. you could be getting that ossilation in any of these.
here is a good test. Using your FX return as a guitar input, turn the control for FX all the way to 100% wet and jack your guitar in there. When you do this, the only active controls will be OUTPUT VOLUME, the PRESENCE of the selected channal, and the EQ if you ahve it engaged. You can crank up the output volume a bit and play. If you dont get any of the ossilation, then you know that the problem has to be coming from further up in the preamp. the FX return, which uses half of the V3 tube, patch the signal in and puts it directly into the power amp section, thus it is bypassing V1,V2,and V4( which comes after it numerically, but really is before it in the signal chain), and v5 the reverb tube.
IF YOU DO HAVE OSSILATION while patching into the fx return, then i bet it is most likely ossilation coming from the phase inverter. If ossilation is occuring at this point, as traced above, you signal is really just running from V3 into V6, and then to the power tubes, so you will know where the problem is confined.
So what do you do with this? If you can narrow down that the problem exists high up in the preamp section, or way down in the power amp section, you can save your amp tech some prodding time, and save your sself a lot of money for him not having to do it.
let me know what you find from the test above