From the descriptions above, IMO the preamp is off the hook- this is a problem with the power amp.
Also based on these descriptions, its my theory that the amplifier has a problem with noisy rectifiers, which is getting picked up at the input if the source impedance is too high. This is why it is quiet with a shorted input and a 1K resistor, but not with the Dude (which has a relatively high output impedance as the volume control is at the output of the preamp), and likely a few other tube preamps as well.
Noisy rectifiers can be tricky as the commutation noise they produce acts very much like RF (which *is* part of their noise component), in that it can easily get to other parts of the amplifier, and use input connections as an antenna.
However this is only a theory, another possibility is a layout problem where the AC power is somehow too close to the audio input. Either way though- things are pointing at the amp.
Also based on these descriptions, its my theory that the amplifier has a problem with noisy rectifiers, which is getting picked up at the input if the source impedance is too high. This is why it is quiet with a shorted input and a 1K resistor, but not with the Dude (which has a relatively high output impedance as the volume control is at the output of the preamp), and likely a few other tube preamps as well.
Noisy rectifiers can be tricky as the commutation noise they produce acts very much like RF (which *is* part of their noise component), in that it can easily get to other parts of the amplifier, and use input connections as an antenna.
However this is only a theory, another possibility is a layout problem where the AC power is somehow too close to the audio input. Either way though- things are pointing at the amp.