There are several factors that cause differences between studio vs. PA mixes, not the least of which is the fact that studio monitors are designed for sound reproduction, and PAs are designed for sound reinforcement. Two different animals.


Another factor is Peak vs. RMS level. One can normalize either or both, but in doing so, will likely compress the dynamics right out of a mix. If one only normalizes Peak to minimize this, the RMS (and certain frequencies) will likely be all over the map, causing different tracks to jump out or drop out. That is precisely the phenomenon that the OP is experiencing.



“I've tweaked the volumes on certain instruments/styles and use MP3Gain to supposedly make all the MP3 volumes fairly equal, but find that on a gig, I have to scramble to raise/lower the volume on a particular song, or turn down the bass, etc...”



In the real world (not the theoretical one), one cannot “... adjust the mix.... bass, highs, level, to any club/venue situation easily with the tone controls on the PA mixer board...” without interrupting the performance; never a good thing.


As stated before, the only viable solution I have found is to do final mixes through the PA, in context with the other songs, by using your ears, not any “gain reduction” program.