I have completed a test. Using a mix I tested 1. Via the Revox tape amp alone and 2. onto tape and then picking up the tape monitor and out via the amp.
1. Amp alone - I noticed a roll off of below 50 Hz and some roll off in the high frequency range which I could quite pin point
2. Tape monitor via amp - The low end roll off increased to below 90-100Hz. Top end stayed roughly the same with some high end roll off and there was a low mid boost around 100 - 220/250Hz
I didn't push the signal too much as the difference between being clean and distorting was a very fine thing. I used an Ex BBC reel of Ampex 406 mastering tape for the test.
I am not sure I would use this on a master mix like this, however I will also test recording via the tape monitor to see what that might do instruments.
I believe I could get a similar more controllable result though sending the signal via my ART MPA PRO II tube preamp which also has line-in and variable impedance. I can control that very well and get a touch of distortion without everything falling apart.
Cheers - Mark
PS - In past times I would have been using a pair of multi channel DBX compressor/exanders between the desk and the tape machine which always worked seamlessly so I never noticed too much difference between the recorded and played back signals so this is a first for me, putting an untreated signal onto tape and pulling it off again.