Two cause; if the noise is variable, it is in the gain stage and may not be a tube, especially if it is both channels. I doubt both tubes went bad. But, you say it is constant.
It sounds like the noise is after the gain stage since it is constant volume superimposed on the signal. That would tell me it is a final output tube(s). If it is ONE channel, switch the two output tubes to verify the noise changes channels / follows the tube. Or, did the noiser tube change sides?
If it is thermal shot noise or plate grid voltage noise that you hear it won't amplify. Be sure you know which tubes are in the final output stage (past the volume control).
Experts are here to help if you give the model number of your pre amplifier. And, the manufacturer can quicky identify your problem in short order if it is a tube issues and not the high-voltage stage driving the tubes (that will need service).