One thing I discovered about MJUC and that is to set the cal level. It is up the top right. It sets the VU meter response level wise and the signal level through the compressor too. Mine was on -18 but I have been working at -14 instead. Once I set that to -14 the input output levels changed and it seemed more correct for -14.
(these are all rms readings) I have been using no drive, opting for total clean sound. I did go back to the Model 3 in the end because the client could hear the compression as subtle as it was from Model 2. In model 3 there is just no hearing it.
The side chain filter needs to be thought about too. If you raise it then it does not respond to the frequencies below where it is set meaning you will get more low end through uncompressed. I preferred the sound however using a HP filter to remove unwanted sub bass stuff first before hitting the EQ even. That meant the SC filter could be dropped right down or off so the low end was having an impact on the compressor too. If the sound hitting the compressor is well balanced then it is a lot happier in general anyway.
I have finished mastering this whole album now and it has done an excellent job on it. Sounds killer everywhere and in the car too!
Yes Mixbus is a different thing. I was able to do what I needed mastering wise with MJUC instead. Sometimes Mixbus does not always work perfectly for mastering either but that is another story.
I also have Klanghelm SDRR too and that is pretty nice at what it does as well. I tend to use that on buses just processing part of a mix though, not for overall mastering.