I've been using the Xotic EP boost lately. Its very clean and kind of tailors the sound a bit by rounding out the highs and lows, but it doesn't really boost the amp too much unless you want it too. Even then, it doesn't add a lot of gain... just volume and tasteful eq/dynamics. It makes certain elements of my playing pop out a bit more... which is nice. Its actually really cool in the loop with a clean delay to give a slight solo boost + delay if you want. Once you get into really high gain modes, it doesn't have as pronounced of an effect. Luckily, its one of those "always on" type pedals if you want it to be so you can move it around and see where it fits best. If you want a clean boost with just a bit of eq, I'd go with this one over other ODs. I haven't really found a gainy OD that I've liked with the Mark V (Green Rhino, Pork Loin, TS, MXR Distortion...).
I do have a few fuzz pedals in the line though... If my amp made that tone without pedal help, I would think it was broken!