Thanks for the suggestion. Since my 'computer room' and 'music room' are one and the same, I have server-grade power conditioning and battery-backup UPS units on every outlet in the room (which are on 3 different MAINS power circuit-breaker circuits), and the server rack that holds most of the equipment also has power distribution that further smooths out the electricity to pure-sine wave.
All of these also have EMI/RFI filtering built-in.
In the past I once had a problem with cheap CFL fluorescent light bulb, which as being cheap had poor shielding, and it took me ages to pinpoint the cause via process-of-elimination, and I have those clip-on ferrite filters on both ends of almost every single (of more than 100 different) cables for all of power, audio, and data.
So I am no stranger to the problems you are mentioning, and sometimes still get interference (sometimes ground loop hum), but I have not tested recording with the new iRig via USB yet on another device...
I will report back once I can, and I will also be going through all the wiring in the next week so hopefully will find any gremlins if they exist.
The only way really to do more, is to wallpaper the room with chicken-wire and make a Faraday Cage. but I will be moving soon, so that might have to be in the next home.