I have to chime in on this thread. I can't comment on the original question, but I have a couple of F-50 observations. I picked mine up this July after waiting over 3 months for the custom build (wood & wicker). The reason I got the amp was to avoid having to haul my old DC-3 plus 2x12 to gigs in order to be heard. I play tested the F-50 at the GC, and felt it would handle most of my gigs no problem in the volume department.
I was wrong. The amp is bone crunching loud on channel 2, but channel 1 is barely (if at all) louder than my DC-3, which is rated at 35 watts, so I still have to use the 2x12 for most of the rooms we play. The observations made by others are spot-on: the taper of the output volume on this amp is WAY different from most Mesa's I've played. It comes on strong VERY quickly. But once you get mast the half way point on the channel masters all you're doing is adding output tube saturation to your sound, which to my ears is perceived as a slightly more clipped and compressed sound. And it does get a tad bright. However, you'd be hard pressed in most situations to get past 10 or 11 o;clock on the drive channel even without the contour engaged. It is ridiculously loud. Having said that, I do feel that to get the most out of the drive channle you should be at or close to 9 o'clock. The difference between 8 and 9 is huge, and the difference between 8 and 10 is monstrous.
It's a great amp, but I'm a little disappointed that it seems to run out of headroom in the clean channel as early as it does.
That is all.