Shorting Plugs and Non-shorting Plugs - Wothwhile?

Not for your problem. Try disconnecting cable tv line and ground and see if that makes a difference. Have you tried the preamp headphone combo at another location to see if it's still there? Take it to a friends house that doesn't have a humming system. If it hums there, something internal is defective or a poor design.

Have I got a short memory or what! LOL.

I thought you had cured your hum issue by letting your pre-amp warm up. To isolate your problem to the malfuncion of something internal or interface with something external disconnect everything from the pre-amp. Take it to a part of this house which has power outlets on a different circuit than any major power consumers, or better yet, any at all, plug it in. Listen - what do you hear? Go back every couple of hours and listen. If you hear nothing the hum is caused by something external to the pre-amp, most of which have already been sugested, or if you hear something its probable that there is something wrong inside the pre-amp, like a failing cap or something similar and needs to be diagnosed by a techie and fixed if possible.

Good luck.

Disconnecting the preamp, and temporarily relocating is the only thing I haven't tried.

I thought I had cured the problem, too, by leaving the preamp on, suggesting that the problem was thermal in nature. But no. One of the things that makes this problem so hard to solve, is that it's sporadic. It makes it very hard to find a hard fault that can be isolated. Every time I think I have it solved, it comes back. Maybe, if I have about 45 beers, I won't care...

If the pre-amp hums with nothing else plugged into it (except headphones), then you probably have a leaky capacitor in the power supply.

The only other thing I can think of is that you're equipment is right next to an extremely high level of EMI or RFI radiation.

This is easy to diagnose. Just set a portable AM radio next to the disconnected pre-amp and tune it off station to static. If the pre-amp and radio hum together it's not the pre-amp.

Now unplug everything from all your 120V electrical outlets and then plug in only the pre-amp and a clock radio into the same outlet. Tune the radio as above and listen for the hum. Both together means it's not the pre-amp. Just the pre-amp means it's the pre-amp.

Final final test, buy an emi/rfi filter at your local BB, Walmart, etc. Try it and see if the hum stops. If it does, keep it and call and see if you can find the cause of the interference. If it doesn't return it.

I guess it's the preamp. Just the preamp hummed using the radio test as described by Dan Myers. I like black and white tests like he describes. I can make a determination, and move on to my next step. Whatever that is. It just may be a new phono preamp, plugged into the (silent) tape inputs.