Hey, I butted the M6 up against my B1500 this weekend, because for my gigs, the M6 falls into the class of "big/ heavy/ loud" for the (admittedly) relatively few gigs I need gargantuan power. Yeah, I said gargantuan.
"You know, I've always liked that word... "gargantuan"... so rarely have an opportunity to use it in a sentence."
Anyway, IMHO despite the published specs (which is a whole separate debate anyway) the B1500 and M6 are pretty comparable in horsepower on a gig... at least subjectively through a single 4 ohm 410 (in my case, Epifani S2 UL410... currently, my all-time favorite bass cab). The key is all that headroom built into the "600W" M6. Wow.
So at least for me, the power is comparable. However, the tone is not.
I'll be the first person to admit the B1500 is one of the best values on the planet... bar none. Great tone, great power, great quality, fantastic price. However, the M6 is noticeably better sounding. Yeah, yeah, I twisted knobs, but at the end of the day, I plugged the M6 in, just barely tweaked the bass and treble a touch, never messed with the mids, and didn't need to touch the profile switch. Done. Awesome tone and scary badass power. I switched to the B1500 and started immediately twisting knobs to try and get the same tone. Not happening. I got "good enough", but I missed the M6. There's a clarity the M6 has the B1500 doesn't quite do well. If I tried to push the high mids (the treble is voiced too high for boosting... it's mostly useful as a cut), all I got was more "clack and clank". The M6 has a nice sizzly upper-mid/low-treble response that gave me great fingerstyle tone (what I do 90% of the time) right off the bat. For slap, I just dropped the treble/PTC on the bass a touch, done. I always like to have a bright full tone at the amp/cab, and then "tweak" using right hand position/technique, blend control, and eq/tone on the bass for the rest.
Also, there's just no way to get the same kinda' growl when you don't have a tube helping to define your tone in the preamp. The drive control on the B1500 does pretty well for what it is, but the M6 just simply reacted better when pushed hard on the input stages.
At least for my personal tastes, the M6 is the perfect marriage of all the best qualities of an SS head and a hybrid head under one roof. I gotta' get me one-o-deez!