I made up an adapter cable from the mic processor to my Heil Pro-Set Plus and another to the mic input on my Kenwood TS-2000 and started testing on the air. I quickly discovered that while some bands were OK, others were causing some RFI on the transmitted audio. Depending on which antenna I used, the RFI ranged from barely noticeable to so bad that it blocked my audio completely. A couple of cases were so bad that even keying the radio caused RFI feedback which continued until I unkeyed.
Every few months I would get an idea and try again. Modern HF radios use BALANCED inputs, and so I went through and made sure that nothing was pulling the differential pair to ground. I didn't find anything, but at least I had eliminated that as a possible cause. Maybe I had faulty bought equipment? I tested using some other audio gear and found that the problem shifted around; some bands got better, others got worse. I gave up for about nine months after that.
It's a little embarrassing to admit that it took me so long to figure this one out, but I'm really pleased to have finally resolved this one. I'm looking forward to finally getting some use out of the mic processor I bought!