Huh! OK- yes, its very possible (and with this new information, very likely) that indeed the preamp is simply passing the LF noise along from the source. However there is a feedback mechanism that is also in play and its hard to know if that is the preamp or the CD player.
BTW, the preamp is not supposed to have anything in it that would 'correct' this!
If it were me, it would re-install the 100K resistors, and then run the preamp with the source, with the volume all the way up, and see if you can provoke it. Once it starts I would disconnect the source and see if it shuts up.
It seems highly coincidental (and a bizarre explanation) that you would have this phenomena in the CD player and it not also be the source of the problem! So right now, barring further information, in my mind the preamp is tentatively off the hook and the CD player should be under scrutiny instead!
FWIW I would not expect any LF noise from the digital system unless the tubes you are using are noisy/defective.
BTW, the preamp is not supposed to have anything in it that would 'correct' this!
If it were me, it would re-install the 100K resistors, and then run the preamp with the source, with the volume all the way up, and see if you can provoke it. Once it starts I would disconnect the source and see if it shuts up.
It seems highly coincidental (and a bizarre explanation) that you would have this phenomena in the CD player and it not also be the source of the problem! So right now, barring further information, in my mind the preamp is tentatively off the hook and the CD player should be under scrutiny instead!
FWIW I would not expect any LF noise from the digital system unless the tubes you are using are noisy/defective.