This thread needs "Carbine" in the title so we can actually find it in a search, just sayin....
So, I just took delivery of a new M9, serial # in the 1000's so up to current spec. I had one of the first Fathoms a few years back and really dug it - sold it to get an M9 but it took me a while to get back to it. Anyway, some thoughts...
Construction:
The case is much nicer/heavier than the Fathom I had - I actually complained on my warranty card back in the day that the Fathom's was way too flimsy. Don't know if the current M6 is better, but the M9 feels nice and solid. I don't notice any noise/hum from the amp - nice and quiet, and the fans are fine as well. Ventilation is great - the amp stayed nice and cool during a good hour of max power pounding at 4 and 2 ohms. It behaves well even with master all the way up (running gain around noon) - it doesn't get any louder past noon, just dirtier, but it doesn't clip nasty or loose control of the damping at the limits. Also, the power light doesn't dim on transients like my M6 did. Running it at 2 ohms, the tone is a bit dirtier and there isn't any more power, maybe at bit less. Unscientific, but I was able to get it to pull about 4.5 amps max in 4 ohm mode, at bit below 4 in 2 ohm. It's cool to have the ability to run into lower impedances if you have multiple cabs.
Output:
As someone who likes to play way too loud, I find the unit has ample volume - gain and master at noon on this head puts me about where I like to be volume-wise. I remember the old M6 being very loud - this head is certainly beefy, but max output doesn't seem as impressive (to me) because I've been playing though a bridged CA9 and now PLX 3602 for the last several years, and my reference point is admittedly way skewed... taper on the master is much less abrupt than the M6 I had.
Tone:
It doesn't seem as bright as I remember the M6 - the standard flat voicing works very well with the rest of my gear. I like positions 2, 3, and 5 on the mid selector. The subtle mid scoop on position 2 gives a nice, deep, round sound; position 3 is a nice neutral voice, especially if you are using the graphic EQ, and position 5 gives a nice low mid push for some cool rock tones. The graphic EQ works great, and is nice feature to have though I can get where I want to go without it. I briefly played with the compressor and it's not my thing, so can't really comment - I will likely leave it off.
Love the tone through my JGR 15/8's, and both the Berg NV412 and 610. The 610 especially benefits from the native voicing of the head IMO. Should also kill through my 1810 when I get it back together.
Here's a video:
http://youtu.be/Pk08vBGHYkQ
Here's proof of life: