To add to Saluki's comments, the BB750 was definitely more of an old school rocker's dream... especially with that classic distortion channel. You want that grit and nasty growl, the BB is yer man. The M6 simply won't distort like that. You can get it to that "tube knee" where it'll have some extra growl and also some of those nice tube harmonic overtones, but still very clean compared to something like the BB750. It just doesn't do the dirty grit thing. However, that's good for me.
On the tone itself, like Saluki says, it was noticeably bigger and brighter sounding, and didn't have quite the same emphasis through the low mids... at least flat. However, and I demonstrated this for Saluki as well, by dialing up just a little of low mids on the bass, I got darn close to that same sound... in fact the biggest difference is the M6 still had more high end, so I would also have to drop the treble a bit to really cop that tone. I was also able to just tweak the M6 to get there, but what I thought was cool is, with the M6 set basically flat, it responded so well to a little bass EQ I could pretty much do anything I wanted right at the bass. Also, I could definitely tell there were more tubes used in the BB. It wasn't quite as "quick" as the M6.
So here's the way I wrap it up...
Except for the distortion thing, as I've posted before, I could get close to a BB750 tone from the M6, but not vice versa. The BB750 just doesn't have the dynamic range the M6 has. The BB750 does what it does probably as good as any amp in it's class, but tonally, the M6 wipes the floor with it (IMHO!) because it has so much of a wider palette, and at least subjectively, has at least as much headroom.
Disclaimer: Obviously, I put the M6 above the BB750, but that's also ONLY because of what I personally look for in an amp. I hate distortion... I'm really more of an SS person, but at the same time, I dig some of the things tubes do for tone. As I've also said before, it's like someone took what I consider to be the best in tubes and SS and built an amp with both. I'm absolutely positive there's plenty of great bassists out there that would be my polar opposite and be justified by their personal prefs. This is not a slam on the BB750... just not at all MY thing, whereas the M6 very much is.