*** Small glimmer of sonic light at the end of my tunnel - I ruled out the speakers, because I put a small mixer into the system in the direct (dry) inputs and the signal was fine.

That means that the issue is the vocal processing unit, itself. This is NOT good news, but may provide a workaround. There is a dynamics processor in the mic chain that is far too noisy to be useable at this time. I need to be fair and play with levels, and different mics before I write the obit, but from the hours of tests I've run so far - I believe that this is the enemy area. When I turn off the dynamics processor (compressor/gate) the signal is too weak to get a proper blend, as soon as I turn it to an effective level ... there's the noise. This is the most sensitive of all built-in vocal processors I ever used. As soon as I push a key on the instrument - the gate opens up and the hiss is back ... even before I sing a note.

I know how tricky compressors and gates can be, but I'm very tech savvy, and have used these devices for years and years, so I'm bold enough to say, this is NOT user error.

Any other 900 owners feel this way? Am I alone in this quest for quiet, solid, beautiful vocal sound from a built-in unit? Help me Obi Wans ... you're my only hope.
In three hours, I'll be on the job with this hissy beast, so look for reviews after midnight EST US.