Mike, thanks for those inputs. But I’m puzzled by the fact that the three tube types you mentioned use different filament voltages, among various other differences in electrical parameters. The 45 has a 2.5 volt filament; the 101D has a 4.5 volt filament, and the 300B has a 5 volt filament. Not sure how the "101D/45" setting you referred to could work with all three tube types. Although I’d imagine that a setting intended for use just with a 101D, that applied 4.5 volts to the filament, could work with a 300B in a reasonable manner, at least in a line-level application such as a DAC. It definitely wouldn’t be healthy for a 45 though.
Regards,
-- Al
Regards,
-- Al