Shorting Plugs and Non-shorting Plugs - Wothwhile?

You don't have much to loose by trying, they can reduce noise from a source external to the pre-amp. And have worked for me. But so has a good power conditioner or a dedicated line. You won't harm anything by shorting out inputs. But, unfortunately sometimes 'hum' is intrinsic to the amp or pre-amp itself (have you listened to the amp itself without anything plugged into it?).

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.