What is the bestObviously the default value (PreAmp=128 in mptrack.ini), since that's probably what most people use.
I disagree completely.
As stated earlier, using a preamp value of around 33% gives the most balanced feel. But even better is to not use it at all by using mixmode RC3.
The science behind using 33% value is based upon that somewhere in the code when using RC2, 33% is set to be 100%. This can be noticed when using DX effects in your song. They're set to the max, which can be influenced by the preamp setting. Going lower than 33% and you'll see that it never peaks out to the max. Going over it, and you'll hear that it destroys your audio.
I've found that the same can be heard a bit with normal plugins too, but its not as clear as with DX effects. For that reason, I brought this up with Rewbs and Atlantis some years ago, which is when the entire mixmode discussion started.