Thank you for answering!
OK, the pdf you linked does sugguest (as you somewhat gave a hint to) that a minimum of aprox. 160kbps is needed to not affect the research results.
I am still a bit insecure, though. There seems to be a special case: http://www.pleasurizemusic.com/en/why-do-data-compression-processes-mp3-example-strongly-affect-loud-masters-no-headroom
What is described here, is that different from the loudness war phenomenon (which will not play a role here)? If the studio master has already been made very loud, it will get problematic after compression. So let's say from the tape, they made an audio CD setting the loudest point as high as possible or even upping the most silent points of the tape. I guess when the MP3 seller / the label then compressed that to mp3 you get a similar phenomenon like in loudness war (I guess that was what you were referencing to radio broadcast situations etc.).
Well, ok. I can never know, what the studio guy did to the tape. When the conductor conducted from pianossimo to fortissimo and the studio guy just equalized that, I can't do anything about it. I probably won't even know, if he did change the conductors intents.
Apart from that is the loudness war stuff been done to classical music, too? Maybe in a false attempt to make it more impressive to the listener?
Another thought: according to the pdf, in my tests, could I go so far and even compare 256kbps mp3s (as I guess that is the most likely to be found) or 320kbps mp3s with FLAC/ALAC and wav/aiff? Like conductor 1 as MP3 and conductor 2 as wav (which would be derived from FLAC) and compare them?
Maybe I should even compare mp3s of the same conductor and same recording against each other, maybe I find that different mp3 sellers used different quality compressors (given that all use 256mbps).
Since I have a Mac and Sonicvisualizer does only take wav I will also have to look out for something that can convert FLAC (PC) to wav and ALAC (Mac) to wav and maybe even aiff (Mac) to wav (PC).