Something I just realized that would have greater bearing than what I said before is what level is being calibrated to 75dB at 0 on the display? For instance if you use Avia or DVE to calibrate a system, they output a pink noise and have you set it to 75dB. But that is not at full output, it allows for peaks well in excess of 100dB. Off the top of my head I don't know what output the 990 would use as its noise level, but it probably wouldn't be 0dBFS (decibels Full Scale, the loudest one can do in a digital signal). But likely there is some headroom involved.

As for what volume change can be detected when you adjust one of many channels, there is not an easy answer. It depends on the material you are listening to, and the person listening. It should be well under 1 dB, maybe as little as .3 dB, but there are a lot of variables involved.

The easiest to hear, and hence the smaller change needed to notice, would be a sound that is recorded well and the panning location can be heard, especially something like a vocal in the center of the image. As soon as a small amount of adjustment is made that sound will seem to move toward the louder speaker(s). Similarly if you add a slight amount of delay to one or the other the sound will seem to come more from the side without the delay. This is a function of how our brain works.

Now if you have a system that doesn't image very well, or a recording with issues so that the imaging is rather blurred, you can probably make a bigger change without noticing. Unfortunately I can't just say something like, 'oh, yeah, it is .564 dB.' Sorry.

Oh, and for the very last part of your post: If the 990 is truly working in dB (which it sounds like it is) then yes, no matter what the master level is set to, if you adjust 1 dB, that channel will change by 1 dB. It only gets weird with equipment that uses arbitrary and/or weird level indications. My current receiver behaves like the scale it shows is linear. That can lead to some real confusion!