A 3-prong to 2-prong adapter will probably still provide a ground, if its metal grounding tab or grounding wire is connected to the screw that holds the faceplate of the outlet in place, and provided that the wiring within the walls is enclosed in metal conduit. If you have a multimeter, or a neon-bulb AC tester, you can verify that by checking for the presence of 120 (or so) volts AC between the AC hot pin of the outlet and that ground screw.
From your description of the symptoms, I don't think that AC grounding or feedback have anything to do with the problem. And the fact that it works OK at very low volumes would seem to say that the problem is NOT upstream of the volume control (meaning that it is not being caused by the source component or its interconnect cables, or by preamp circuitry that is ahead of the volume control). Although it would probably be worthwhile to double-check that by trying a different source component, such as the cassette deck.
Some things to consider:
Are you certain that the new biwire cables are connected correctly, and that there are no shorts in the connections or in the cables themselves?
Are you sure that when you heard reasonable sound at low volume settings that you were getting normal stereo separation? It's conceivable to me that a short between channels could produce reasonable sound at low volumes, except that it would be mono, while causing the symptoms you described at higher volumes.
Along the lines of one of Lloyd's comments, but specifically with respect to the power amp, I would consider the possibility that the amp's input jacks may have been dislodged as you were connecting things, such that they lost their connection to the amp's circuit ground.
And, yes, it does seem conceivable to me that a defective tube, or the wrong tube type, or a tube placed in the wrong socket, could account for what you are describing.
Regards,
-- Al
