So...I had to try again. And thus, I rebuilt the pink noise feeding two auxes (prefader of course so I could pull down the fader of the original software instrument with the pink noise generator on it).
Just as a control, I put the gain on the second aux, inverted polarity and it all summed to zero. Good.
Then I did the same thing as I wrote before with the EQ inserted, no bands active, but with +6 on the EQ's master gain. Inverted the second aux's polarity and....it bleeds through. Certainly not a lot, about -48dB or so from the normal sum.
Then, I tried it with the make up gain on the Compressor plug-in. Obviously no GR happening, just gain. The exact same.
Finally, I raised Aux 1's fader to +1dB and set Aux 2's fader at -5dB (still a 6db difference to counter the EQ or Compressor's gain) and ....it summed to zero.
I'm sure everyone in the world has better things to do in their life, but it would be interesting to get confirmation. I can't think of anything that is unique to my set up, etc, that would cause this to happen.
"To measure is to know" - Lord Kelvin