Interesting question, I'm also experiencing that all the time. I don't care so much about it when using programs that are based on digital algorithms - but as soon as I use samplings of mechanical devices (especially spring and plate reverbs) I think it's important to work in Nebulas gain 'comfort zone' to get all the different distortion layers working correctly.
It's difficult though - aux reverbs combine many different sources and if I gainstage correctly for let's say a background pad, all snare hits will usually overload it.
I think we are just trying to formulate a helpful response ... that, and we forgot the original post
The basic premise would be that 'audio' can cover the full dynamic range, as we know.
Proper gain staging sometimes require a 'creative' solution though.
Example ... you could place 2 'gain' plugins [bookends] around the Neb reverb, with counter settings that would boost going in, and cut on output [or use Neb's builtin IN/OUT gain. If you add +3 or +6, just cut that at the output. This would maintain the same 'blend' level while allowing internal gain to be controlled as desired.
Well, if we think of Nebula as analgoue device, than RJ's comment makes perfect sense.
On an analogue console you will have an Aux Master Level for the bus that collects all individual Aux Send to it, and the level will affect what's going to your FX unit. So you can always regulate the levels from there. [This in your case would be some sort of Gain insert plug-in on the Aux, but pre-Nebula plug-in]. You would also probably have input trim on the FX unit too [in our case it is the input gain in Nebula].
Than you will either return the FX into a dedicated Reverb/Echo Return or your Channel/Monitor path of the console. In most cases all these returns will also have gain control and even EQ.
So it is sort of the same thing ITB - you would be gain staging it before and after the effect. Ideally it should all start with the source - in this case the individual sends from each channel.
I would start with the loudest part of the song and the sends from the channels that would contribute the most to the Aux Master. I'll send a rought balance to it and adjust the gain. Than you can always bring down the Fader of the Aux in your DAW, which in this case is what would have been the gain of your Reverb/Echo Return.