Since Nebula is intended to capture those devices at the highest level of integrity it makes sense to leave the gain structure of the various samples as they are (as altering them would indelibly alter the programs themselves).
There are also programs which are very carefully crafted (after the sampling process) gain-wise in order to do specialized jobs (reel to reel programs come to mind); they also don't fit neatly into the -18 = 0VU box sometimes.
This is no different than the kind of pain someone with a good analog studio and lots of hardware would have to go through (except, theoretically, they would also have the pain of hardware meters).
What people have suggested is that the information about the optimum level ranges and what not for each library/program be compiled in a single place in order to help ease the pain.
It seems like we are running parallel threads now...
Fact of the matter is that I've asked and received the proper information from Micahel (cdsoundmaster) as to the way R2R should be set for various mixing situations to prevent (additional) gain steps. Below are the instructions as were e-mailed to me:
Here is the main thing that matters in this case. Each R2R program is sampled based upon the actual tape machines calibrated levels. The maximum amount of saturation is edited to +0dB so that if you use the loudest file, it is the same as using the loudest level to tape at that program setting. When basing this on metering rather than the level of the hardware input/output, it is a matter of adjusting the Neb input/output gain like you are describing, to essentially trick the machine into thinking it is working the same way with lower settings. In the real world, it would be the equivalent of raising your source up from -18dB to 0dB.
I recommend that you increase the volume in your R2R settings to your absolute peak volume. If you meter by an average of -18dB and a max peak of -6dB, then I would increase your input gain by 6dB and lower there output gain 6dB. If your max peak is never more than -12dB, then you can raise the Neb input gain by 12dB and lower the out by 12dB.
To do this, go to your GLOB, and you will see "padin" and "padout". Use pad in to increase the input level by the gain that is desired. Adjust the output for the same amount of reduced dB. When done, I recommend going to edit and naming the program something new and choosing "save as". That way, you can make a set of programs for yourself based on analog calibrated digital translation metering like the K-System. If you ever needed the same programs for +odB they would still be available. Also, editing these programs this way for your own use does not take much room on your hard drive so it is practical.
The R2R is made for general use (mixing and mastering) and is set in a way that prevents unnecessary user errors. I think users can use the information available to create their own R2R programs which are calibrated to suit their needs (and taste).