on a clean slightl mid-scooped amp the OD-3 is a marshall in a box.. It retains more highend than the sd1 and it boost low mids giving the amp and more low mid pushed marshall sound.. The BD-2 does not boost the lowest mids like the OD-3 and it has a ton of high end.. and no mid boost. ,, I find the tone pot on the BD2 a bit weird.. With a clean amp and single coil pickups I need to put the tone pot below 10 oclock.. or it boost the highs with a overdriven sounds.. if you set the tone pot at the right setting the BD2 is the most transparent.. (does not alter the EQ) ,, though takes of a little low end.. The SD1 works best as a boost into a slightly driven tube amp or into a BD2 set to mild gain..