i have never understood the use of a tubescreamer to boost a marshall or 5150 etc.
This is what I use my OD-9 for. I was a bit skeptical about it, although I had heard videos etc with a TS-9 through my amp and cab combination (Peavey 6534+ & Orange PPC412) and noticed the audible change in character (not just volume but EQ) using the screamer as a boost. It EQs, compresses, and boosts the signal going into the front of the amp. I really didn't want to rely on the 'drive' feature of the pedal, but rather to provide a signal to the preamp that would then produce a change in sound via the gain structure of the amp ( I hope this makes sense). The Peaveys can do with a bit of tightening up in their sound, especially at the low end. The OD-9 does this for me, both with the Explorer (A-Pig set) and SG (Cold Sweat/A-Bomb). Actually I couldn't be happier with this pedal as an effect. Like others have said though the differences between overdrives are so slight it was largely a matter of picking one and going with it. The idea of going with a version of the original TS-9 with true bypass appealed to me, just in case I didn't want that buffer effect sometimes with this pedal, which I use most of the time. So I got the Maxon. But I could have just as easily got an Ibanez or some other brand. The one distinctive feature of the OD-9/TS-9 that people mention, the slight mid hump, I thought might be useful so I went with that.
With the A-bomb guitar (which is much more middy) I set it as follows:
Drive 0
Tone 0
Level 8
With the A-pig guitar I compensate for the extra bass a bit by moving the tone knob up some if I need it. For example, if playing Venom I might move the Tone up to 5 or 6, although I could definitely set it higher. The Drive is always on 0 and the Level on 8.
It's not just mids, there's a bit of compression as well.