Hmm. Well, I find it interesting that you have issues only with one device. Then again, perhaps the other devices are less susceptible.
As for my "crazy" grounding solutions, well, if you have an electrical problem in your home (maybe on the outlet you tried running the Magni on), then its not crazy to suggest you contact an electrician. I was just trying to save you some money, by suggesting some trouble shooting methods, before you go spend money on an electrician. Then again, maybe you don't mind the possibility of having your wires crossed. Literally. Most devices will still work when the black and white leads are reversed behind the wall socket (lights don't care, nor do lots of cheap appliances). And lots will work without a proper ground. I just wonder why you look for advice online, then call it crazy when someone takes the time to suggest something to try. Whatever. Good luck.
I apologize if I sounded off-putting with my comment; I didn't mean it to come across that way at all. The "crazy" description was intended tongue-in-cheek, since I would be a little worried about electrocuting myself with any method involving running new wires for my equipment.
Perhaps manually running ground lines isn't as dangerous as it sounds, but I'm not anywhere near confident enough in my electrical skills to feel safe trying such a solution personally. I very rarely communicate through forums, and apparently I haven't mastered construing the proper tone through messages.
The main reason we haven't tried to do anything about the old wiring in our house is that, as I mentioned, it's never had an impact on any electronics before now (or at least nothing detectable). In any case, as I said earlier in the thread, I ended up returning the Magnis for now, since it sounded like any solution for getting one to work in my house would be quite complicated. If I try some other amplification solutions and they all have similar issues, I may come back to the Magni and bring an electrician in to look at the house's wiring, but, as you noted, that could be quite expensive.
I really do appreciate the suggestions that you (and the other users) have been providing, but I'd like to make sure that the issue is more universal before delving deep into any electrical work. My next test is a FiiO E5, to see whether I experience any issues in either a completely battery-powered or a USB-powered configuration. If I could find a cheap mains-powered amp, I'd try picking one up just to see if I experience the same issues as with the Magni, but I don't think there are any other low output impedance mains-powered amps (most of my headphones are under 80 Ohms, so I'm avoiding the 10 Ohm output models) anywhere near the Magni's price range. Frankly, if other people asked me what cheap desktop amplifier to buy, I would still recommend the Magni first, since there's absolutely no competition for that price/performance ratio right now, and the issues only seem to crop up for (or be noticeable by, still not sure which) a very small subset of users.
I'll report back on my testing of other amplifiers, and, if they're not successful, I may very well pull in my friend who's an electrical engineer to put together the custom-ground setup you described.