ok, so I've got just about all of my questions answered. big thanks to Doug Oade for the quick email replies. He's much more helpful than the Marantz tech support.
as always, any other thoughts, comments, or input is welcome.
Thanks for sharing the info from Doug.
Was the unity gain settings/levels for the PMD661 suggested? I haven't seen actual unity gain info listed for the PMD661 anywhere. This would be good reference info if anyone knows.
no, I didn't ask about unity gain and Doug didn't say anything about it (I basically posted everything from our email exchange).
I have now done a short little home test with the beyerdynamic CK930 > Sonosax SX-M2 > PMD-661 in front of my stereo.
obviously, it's not quite a "real world" test, because the sound from my stereo was no where near concert SPL's. (even if my stereo could reproduce sound that loud, which it can't, I couldn't play it that loud with my 10 month old baby sleeping upstairs).
anyway, I ran XLR line-in to the PMD-661, and I had the PMD-661 on the -18 dB gain setting.
I ran the Sonosax in the high gain range (at a concert, I'm sure I'd be running it in the low gain range).
I generated a pure sine wave and saved the WAV file and played that back through my stereo on repeat. That way, my source would be a constant level and I could easily play around with the level knobs on the Sonosax and the PMD-661.
The Sonosax outputs a max of +20dBu (over the balanced outputs). The "over" lights on the Sonosax come on 6dB before the pre-amp overloads, so it outputs +14dBu when the "over" lights come on.
I gave the Sonosax enough gain such that the over lights were just barely on the whole time, so the outputs were in the +14 to +15 dBu range.
I then set then set the PMD-661 to peak around -2 or -3dB, and the knob was set around 6 at this point.
So, based on my test, I'm confident that the Sonosax SX-M2 > PMD-661 combo will work just fine. If I just leave the PMD-661 knob around 6, and then set my levels using the knobs the Sonosax, I should peak around -2dBFS on the PMD-661 at just about the same time as the "clip" lights come on on the Sonosax. At this point, I know that I'll be close to 0dBFS, but I won't be digitally clipping. I'll know that the Sonosax isn't being overloaded, because I still have 6dB of headroom there. and I know that the Marantz will be able to handle the signal just fine with no distortion at all. Also, I get to use the XLR line-in, which I'm much more comfortable with compared to using the 1/8" line-in2 jack (just because I like XLR's better).