FWIW, i really like the B1 on guitar amps. i think this is one of the applications it shines best in......much more so than it shines in vocals, IMO. i like the B1 about 4-6ft back, and especially in combination with a dynamic (generally) off-axis up on the grille cloth. watch out for phase anomalies (or move the wav file or use a slight delay to compensate post-tracking) and i've yet to have an amp track that wasn't at least usable. especially, as chessrock said, if it's a clean to mildly overdriven amp.
now, when you start cranking that distortion up to shred, the mic you use is gonna be the least of your worries. it'll be more like "why's this sound so bloody thin?" a little distortion goes a long way in front of a mic. double or triple track that part with 1/3 of the "normal" distortion and you'll likely find yourself in a much heavier world. takes some chops to do that, though.
anyway, here's a big thumbs up to the B1 on amps. FWIW, i've always looked at the B1 as an MDC (it's not *really* an LDC) analogue to an sm57.
cheers,
wade
PS--the V67 isn't too shabby on some amps, either.....but its inherent darkness (compared to the B1) sometimes just isn't what i'm usually looking for when recording low-wattage tube amps. maybe for a big, trebly marshall stack, though.....