That's better...

[mdlbear]

After my walk I went back to Guitar Showcase in search of a second isolation transformer, but realized on the way over that I might not need one. Just as well, since they didn't have any on hand. All I really had to do was move the transformer from the office end of the cable to the bedroom studio end, which put a balanced signal on the entire cable run by decoupling it from ground. The result is still noticeable, but not nearly as bad. It's essentially inaudible (at least to these ageing ears) at my usual listening level.

I could do even better if the monitors had balanced inputs, but a quick look at the manual shows that they don't. I'll have to look for another isolation transformer eventually, but it's good enough now.