As much as I ever had liked the SD-1, the OD-3 is a very good drive.
I did change most of the caps in mine though....not modded just better quality parts.
No blending, just adding a gainstage, it is not a wild pedal, sounds abit flat on its own, but works very well in a mix, I use it mostly for lower gain stuff!
Od-3 is great but too flubby in the bass on higher settings. SD-1 level high, gain low in to a plexi style Marshall is gold. Too thin for me into MV Marshalls.
The ZW-44 OD from MXR is awesome into MV marshalls. It's a SD-1 tweaked to have more bass and beef. Also a Voodoo labs Sparkledrive (I use it now) is awesome into my '78 2203. Blend clean boost with an 808/ts9 style circuit.
just be sure what you end up with doesn't rob you low end. The yellow overdrive pedals were known to do this. Try playing a bass guitar through it before buying. With mine, very little bass gets out the other end. I have a few-year old SD-1. I think they've addressed this issue though.
The thing about the sd-1 is that it works great as an almost clean boost with a bit of mid-range boost as well, and therefore helps the guitar tone cut through the mix like a laser for solos. It does help kick the amp into more overdrive, but Its not about bottom end as much. It more like a tube screamer then a distortion pedal. The bottom end gets eaten up by the kick and bass anyways. You only notice it when you play by yourself.