The recommended position for an sm57 on the amp is right up on the grill (but not touching to avoid vibrations), perpendicular, but NOT in the center of the cone. How far off center can change the character a bit, but I haven't had a chance to experiment with that to see what the exact effect is.
I just finished a music recording class at the community college here, and it was a very enlightening experience. This was the 2nd semester after a 1st semester in the lecture class that covered theory from the book. But this one was 90% lab where we set up the "studio" from scratch each week in the classroom, built around a Mackie 8-buss 24 channel mixing board, feeding a 14-track USB interface running into Reaper Software. Using a 3-way mic splitter, we also set up Pro-Tools on another computer running from the Pro-Tools Digio-3 mixer/input device, and a Mackie Onyx 16-ch firewire mixer also into Reaper. Essentially, we recorded each session 3 times simultaneously so that as many students as possible would have a job to do while the band played. Over the semester, we brought in about 10 different acts covering different styles for tracking sessions, and even brought some of the singers back for overdub sessions. We had a fairly decent supply of microphones to play with to mix things up and find out what worked best under different circumstances, so it was a really great opportunity to play with this stuff hands-on without the pressure of somebody paying for that time!
I initially took these classes to supplement my TV/video production tool box, but recording music is something I've wanted to do going way back. In an example of how networking eventually pays off, a guy who I helped with on his short film project from 3 years back is starting up a music studio, and has brought me on board to help him get things situated and for the contacts I've made in the local music scene with my live music videos, so I'll actually get a chance to do some real recording here fairly soon.
To get back on topic, the one mic I own, which I purchased to act as a "room mic" for my live-music TV show, is an Audio Technica AT2035
large diaphragm condenser. I stuck it up high on a mic stand about 2/3 of the way back in the room to supplement the feed I was getting off the board, and mixing just a bit of that actual room reverb (and audience reaction) into the mix has made a HUGE difference in maintaining the live energy that a too-clean board feed often loses. Also being on a very tight budget right now, it's about all I'll be able to get personally for a while. I'll probably use it for some experimental recording sessions with some friends at home so I can get some practice at all this stuff.