Thanks to all for the support and tips. Once I realized I could of course use the DBX EQ to exactly compensate for the level deficiency coming from the RCA path vs. XLR, my blood pressure dropped considerably. And then once I apparently "learned to love" the XLR tonal quality out of the NFB9 and began to think of it as the new "reference" , I was a very different person.
I will indeed follow your suggestion and leave it on (and fed from a CD playing on repeat) for the next week or so with volume turned down when I'm not actually using it to listen.
As far as that fourth screw-in leg is concerned, I can also just put a small folded up piece of paper underneath it, to "even it out" and be done with it. It's a back leg and isn't visible, and as long as I can prevent the unit from rocking when I push the power button or one of the other front buttons that's really all I care about. That might just be a better compromise solution than trying to unscrew it to make it come out a bit... which would then make it loose. Annoying for sure, but I guess I can live with it.
As far as the remote back plate, I figured for sure those four screws have got to be what releases it, and with just a bit more time spent trying to get it off I suspect I will then see some batteries inside... should I ever need to replace them in the future.
Now there is one small adjustment to my "cabling configuration" described previously (in my "breakthrough" post).
I've reversed the INPUT1 and INPUT2 feeds. So now INPUT2 is fed directly from the Smyth Realiser headphone outputs, and INPUT1 is fed from the RCA line output of the DBX EQ. This effectively changes nothing in terms of sound through headphones, and only affects what "input" button I push on the front of the SRM-007 to listen to something a certain way.
But the reasons and justification for this swap are as follows:
(1) SRM-007 always powers on with INPUT1 pre-selected as the default input. So having INPUT1 represent my primary normal listening mode just means that most of the time I won't have to push anything on the amp.
(2) INPUT1 on the SRM-007 also has a "parallel output", which does not require that the unit be powered on in order for the line input on INPUT1 to be routed to the line output from INPUT1 (say to feed some external 2-channel stereo system). As it turns out I have a very nice sounding Altec-Lansing 621 speaker system (a spare 2.1 system that I used to have on a computer, but that sounds very good) which I use as "external speakers" for my HDTV when I want to just listen casually to TV and don't want to be bothered with headphones.
So by having the parallel line output from INPUT1 go to the Altec-Lansing speakers system, this accomplishes what I need. I can use the external Altec-Lansing speakers for sound, with the SRM-007 powered off. That is actually how I had things arranged before the NFB9 arrived, and it was only in my excitement last night that some re-cabling ended up overlooking that requirement.
(3) The cabling involving the Smyth Realiser, DBX EQ, and Stax amp (and now the NFB9) is more complex than needs to be discussed in detail here. But the net upshot of all this cabling and the fact that INPUT1 (and its parallel output) is now fed from the output of the DBX EQ (no matter what the input to the DBX EQ is) means that I now can use the EQ for either of the following input sources:
(a) RCA output of the NFB9 (fed from "processed" optical digital output of the Realiser), and tone controlled by the EQ
(b) "unprocessed" RCA line output of the Yamaha AVR from whatever source device is selected, and tone controlled by the EQ
And both of these inputs passing through the EQ can be listened to by either (a) headphones via SRM-007, or (b) Altec-Lansing external 2.1 speakers.
That's via INPUT1.
And I can still switch the SRM-007 for headphone use, to either INPUT2 or INPUT3:
(a) INPUT2 - pure RCA analog headphone output from the Smyth Realiser, with no EQ tone control
(b) INPUT3 - XLR output of NFB9 (fed from optical digital output of Realiser), with no EQ tone control.
Since I've "leveled" the analog output volume of the Realiser and the output volume of the NFB9, I can instantly and justifiably compare (1) the internal DAC of the Realiser and its RCA analog outputs vs. (2) the external NFB9 DAC and its XLR analog outputs, with no interference of any kind from the EQ.
Ok... now to get down to burn-in, and then some serious listening.
Again, thanks to everyone for your guidance.