Sorry about the zone-related time delay of this reply, Theatre-Hissers...

...But I'm hoping  that something I've noticed in adapting and connecting my huge variety of sources to TBP12 belt-packs may be of assistance.

(Click on the pink pic at left for details of my rig.)

Being all in the bit-mapped digital domain, the processing inherent in these belt-packs isn't the dreaded "Automatic Level Control" compressor of analog gear, nevertheless one of the (very few) artefacts I have regularly observed sounds remarkably like the automatic application of a staggering amount of gain in the absence of "identifiable" (hence encodable) signal waveforms being input at "decent" levels.

If you thing the hiss with nothing connected is objectionable, you should hear the racket when an unterminated XLF (mic socket) to TA4F (belt-pack socket) adapter cable is moved around or "scrunched up", ...it can range from a deafening "waterfall" to an entire avant-garde soundscape !

Now, despite me being an almost lifelong Yamaha fan, their early 01/02 "V" and "R" consoles did have some of the noisiest, dirtiest-sounding and nastiest-clipping mic preamps I've ever experienced, in keeping with the ongoing paucity of their cumbersome, tiny and hard to read LCD menu navigation, ...none of which is relevant to this discussion.

Nor is Yamaha's subsequent reaction in endowing subsequent generations of their mixers with industry-leading sonic transparency and clear, ergonomic operation, at prices relatively far lower than their predecessors.

(If your thinking, however, that I'm hinting at replacing an old 01V as soon as money allows, ...you'd be damn right ! Yamaha LS9's are like 01V's on stereoids, ...only much sweeter-sounding, staggeringly more powerful and far easier to use.)

http://www.yamahaproaudio.com/global/en/products/mixers/ls9/

(When dreaming, ...why not dream BIG ?)

While we're ruling things "out", you've done a great job of proving that mains supply and rack (or any other) wiring also have no bearing on this artifact.

Which brings us back to your fabulous MKE-2 mics, or, to be more specific, quality of sound from their TBP12's and their XDV-75 receivers when they're actually being fed spoken word, as distinct from merely picking up ambient room noise.

Betcha that you won't hear anything other than the performance, the whole performance and nothing but the performance, while an actor is actually speaking.

Nor will any "pumping" or "breathing" of ambient noise be audible between speeches.

It's just that this form of bit-mapped digital input processing renders obsolete the old analog starting-point estimation technique of "preset the input trims till the room noise sounds about right".

Speaking of room noise, I've found it well worth experimenting with the XD-V's "Environment" settings in rehearsal, particularly when given simultaneous overlapping contributions from large numbers of participants. (Yes, I do hate teleconferences.)

I've found "Environment" to have been a really useful tool in such situations, once you're used to adjusting the gains upwards upon switching in the harsher "Speech" mode in the event that you decide to employ it for almost automatic "multi-gated" mixing.

It is, however, much more than just a "gate", ...playing with it being by far the best way to evaluate its dynamic, multi-parametric operation and its consequent appropriateness (or otherwise) for your individual situations.