Another GUI change & the headphone monitoring was working quite well. Not sure why it wasn't tested before, but put the gain to 0 with the headphones connected & the result was what is shown. The noise pegged at -70 instead of -85 & the waveform at 1/65536 was horrendous.
The headphone was a giant antenna, but it was fortunately not noticeable in the spoken test. It shouldn't affect the Zoom replacement, since that's all differential inputs, but it's worth giving the CM109 a try.