Tube pre picking up feedback/vibration?

The resistor change is Audio Research's recommended fix for adjusting volume control setting. Ideally the volume control should be in the 11-1 o'clock position at your desired listening level.

Still not happy with the results of the new tubes. Emailed ARC and they basically said all is well. Now I notice a low hum from one channel and reversing the wires leads me to suspect the tubed. What exactly will a bad tube sound like? Hear the hum with volume all the way down and the cd player stopped. Like I said I reversed the wires to the amp an
d the problem followed the change so I have ruled out the amp. Ideas?

Your cable routing could be causing a problem. Do some 'process of elimination' experiments to see if you can pin it down. If you have a noisy tube, it will sound more like white noise/hiss, not so much hum. A hum in only one channel is often caused by an interconnect passing close to a transformer or power cord.

1st try just reversing the tubes from channel to channel. If the sound does not change with the switch it is not a tube problem. If it does, it is a tube. Replace it.

Next, look at all of your interconnect and power cords and try to keep them well away from each other. If you must have an IC near a PC, makes sure it crosses it, NOT running parallel to it. Also make sure that the IC's are well away from any transformers in any of your components.

If that doesn't do it, disconnect each source first the PC and then the IC's. If the sound goes away your problem is in your source or its connections. If is does go away, then reconnect them one channel at a time and see if it reappears.

If doing all of that doesn't get you a fix, you might consider that you have a cap going bad. Did ARC check out your unit or did you just buy and install new tubes? If the latter then perhaps you need to have a techie check this out.

Short the input RCA jacks and see if the hum is there.
If so its the preamp, if not its your CD player or cables.

Not your power amp.

Yes some tubes are microphonic, some tubes hum.
Carefully swap the L and R channel tubes and see if it moves with the tubes. If not, then send the amp back to Audio research and have them check signal grounding.

I find Marakanetz's post about Audio Research recommending NFB resistor change to be shocking. Changing NFB to tune overall gain can shift the oscillation pole frequency and can cause stability issues.

Thanks...great suggestions . I'll try to get time and let you know what I find.