Help with hum

I have a hum coming through both speakers. It is independent of volume control and completely stops when the preamp is disconnected. Disconnecting source from preamp has no effect. I have tried switching cables and tubes from channel to channel but it keeps humming. I have tried all combinations of cheater plugs on both amps and on the preamp but with no improvement. Any advice regarding cause or diagnosis is appreciated. Preamp is Cary SLP-98 and amps are Manley Neoclassic 300b's.

Thanks in advance,

Removing the preamp stops the hum. All of your circuit design issues regarding grounding and loops are not to be ignored.
However as an owner of a couple of tube pres including a Cary Low end brand. I can tell you that a bad tube can cause a number of different noise issues. I always took the stock 6SN7s out and rolled in a wide variety of old stock. Man oh man I couldn't believe how many fine testing for transconductance when put into a preamp how noisy and bad tubes reared their ugly character.
Thus my question Have you experimented with the tubes? How many hours do you think you have on your preamps tubes?
Obviously you know what I am going to suggest you do. Roll some different well tested tubes into the pre. I know the other sugestions seem more plausible at first glance, but so far grounded has not changed you problem
Hello Dave, I struggled with hum issues for years but not due to any kind of tube issues. My situation: too many different pieces of gear, on too many dedicated circuits, and powered subwoofers. After many trials and tests I came up with the following conclusions. "cheater" plugs will work to reduce hum, even eliminate it- but it's not the safest way to do things because they remove one of the paths for ground, which isn't an issue with regard to sound quality but could potentially cause you to get shocked in the event of a short in the "cheated" component, not all that likely though, especially since your components recieve a redundant ground through the interconnects plugging them together. If you can, try plugging everything into the same circuit, or the same powerline conditioner if you can. Eliminate the possibility of a ground loop that can occur when using multiple circuits, as even dedicated ones can cause ground loops. There are all kinds of devices you can try out there. PS Audio has Noiseharvesters and Quiet Lines. I have used both and they work very well for minimal investment. Often times "mismatched" components will cause DC Offset. That is when the "ground" potential of one component is slightly different from the others in the chain. That will cause ground loops, hum. Jensen Transformers work well as noted by Jea48. I went that route myself when nothing solved my "hum" issues. Of course I had multiple different makers of gear, all being run "single-ended" via RCA connectors, AND i had 3 powered SUNFIRE subwoofers. A recipe for disaster in my experience. The bottom end in my system was mind-numbing, but had hum. Jensen Transformers built a series (4) of ground loop isolators to combat the hum problem. They do it not by "cheating" the ground on the A.C., but filtering the audio signal through a coil, (housed in a nifty little metal box), so the signal, and unwanted hum get eliminated. Now I am not a highly technical electrical engineer type, so I cannot explain exactally how the devices work, but hopefully some technically "saavy" Audiogon member will chime in with their input, explain it better. All I can tell you is Jensen Transformers worked well in my system, did the isolation SAFELY without messing with the A.C., and they were cheap to buy. I no longer have a need for them because my system is run completely balanced from frontend to amps & subwoofers. I have long since sold the 3 Sunfire subs in favor of 1 big beast which is also run balanced. So no more hum. If you are interested, i still have 4 of those Jensen ground loop isolators if you wanted to try them out, see if they solve your problem. I would just send them to you free of charge and you could run them in your system. Call Jensen Transformers first, or go on line and read up on them. I found them to work quite well. Good luck and happy listening. If you want them to try, contact me and I will ship them to you ok?