Edit: You did say you tested them after the oops. For more than just emissions? I still feel like the output tube took internal damage in the process.
Have you...
tested... the output tube after your lapse of cognizance?
Motorboating and lack of volume control could be a burnt up 7B5. ~200VAC (actually probably ~400VAC) applied across the grid and screen will arc or melt them... Plus ~200VAC grid to cathode probably has similar results...