MP2 is basically the predecessor of MP3, and so its sound quality is worse at lower bit rates than MP3's is. However, there's a twist. MP2 will eventually outperform MP3 if the bit rate gets high enough, because of the differences in MP2's compression algorithms. I think 256K is about where the crossover happens, when MP2 starts to sound better, particularly on things which MP3 has always had trouble with, like harsh electronic sounds or crisp brass or drum hits. Go much lower than 256 though, and MP2 starts to suffer and MP3 clearly dominates. Though even then, Opus has gotten to be really amazing and at least at lower rates, squashes them both.
I've never heard of MT9. I wonder if it isn't just a container for stems? That's what it seems like from the description given here anyway.
Make more of less, that way you won't make less of more!
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