Hello!
I use 7-strings tuned to A myself, and assuming that's what you mean by tuned to A, I will start by making some sense of the situation.
A 7-string guitar (or
baritone 6-string, or some poor guitar with guage .014's on it tuned until the strings are flapping in the breeze) tuned to A goes down to 55hz
An SM57 rolls off around 80-100hz
Do the math and what do we have?
Yeah the 57's just won't cut the mustard!
Personally I use Sennhieser MD421's and UHER M538's with a kick drum mic (usually my trusty E602) directly on the speakers with an LDC (usually an AT 3035 or 4033) a few feet in front of the 4x12's (this mic will have to be time aligned in the computer after recording so it doesn't knock eveything out of phase, or sound like your in a concrete cathedral)
Another option if you can afford it is to use Royer R121 ribbon mics (If I could afford them I would be using them) However a 120watt tube amp would probably kill the ribbons at a rather speedy rate! I would also say that an EV RE-20, AKG C414, AT 4033, or any other Large diaphragm mic might be worth a shot as long as they register flat response down to at least 30-40hz.
Basically I am saying:
Put an MD421 or AKG C414 on the cabinet and an LDC a few feet in front of it, then check phase and time align the LDC track.
If the guitar is hitting in the 55hz range then you need mic's (and pre's) that can reproduce those frequencies.
Or just use a POD and laugh in the face of the old school, technology fearing, close minded, "you HAVE to use a 57 through an API to get a great guitar sound" traditionalists, when they say "now that's how a guitar should sound, great job!" because they have no idea you recorded it direct! I say send em home to cry on their Les Paul's and Marshall's with their cave-man reel to reel's, when they finally realize time actually did NOT stop in the 80's LMAO!
Hope this helps!
On a personal note: does anyone have any idea why ESP says their 7-string baritone tuned to B registers to 53hz? (If A is 55hz then how can B be 53hz?)
Maybe something got lost in the translation from japanese to english LMAO!