Well i tried all these out and this is my personal results. Note i was looking to add overdrive, meat etc to my good tone, not use it as the device for achieving my tone. SADDBI: Second time i had tried this, the first time i didnt like it, this time with a better setting on the blend, i.e about 40% i kept my good tone and added a nice drive to it allthough it did pull out the mids a bit. HARTKE VXL BASS ATTACK: Yuk this was the worst, even when off the level knobs etc alter your volume etc.I couldnt get a decent sound out of this very tinny and harsh sounding and it ruined my original tone. MXR-M80: First time i used it i didnt like it, wasnt anything as good as the Tech21, however again with very careful adjustments it did sound better later on. I would say that this is a close runner to the Tech21 but it didnt have that valve emulation thingy happening. FULLTONE BASS DRIVER: Well by far the nicest built, had the new version with toggle switch from comp cut, Fm and Vintage modes.This had a great choice of overdrives and i think if it had had a blend knob it would of been perfect, but it was all or nothing so wasnt for me as my "normal" bass sound suffered at the expense of the overdrive sound.For somebody looking for quality bass overdrive distortion then this is it, somebody looking for a bit of valve growl and grit with their original bass amp tone then its not for them. Conclusion: Tech 21 came out the best by far as long as you work with it and use the blend knob to get the best mix .I used this live and it did really add the right amount of extra kick just when needed and while off it didnt alter my tone at all.