If the sound ping-pongs at high levels I'd definitely read up on 'standing waves', unfortunately I can't say anything without being too technical; also, start looking at pillows and blankets to put on walls in front of, behind, and less than a pace away from the amp.
To be honest, you don't *need* cab excursion, but it sounds fucking good and that's not at all uncommon to have on records. Google 'Slipperman's Recording Distorted Guitars From Hell', read the parts concerning volume, and think about getting a smaller cabinet that won't be quite so brutal when cranked. Cab excursion won't be as loud as you think it'll be - we're talking about shaking a bit of wood ATTACHED TO BLOODY SPEAKERS, not making a refrigerator go on a raping spree - but all the same you'll want to start insulating things before you experiment with that at all.
I'd also recommend getting another dynamic mic like an Audix i5 (which has, in my opinion, a nicer frequency response for high-gain guitar than an SM57 - the 57 starts dropping, at 5dB an octave or so, below 200Hz [between G and G# below middle C] and peaks around 5kHz, which is not a happy frequency range if you don't want to sound like bacon), so you can start playing with dual-micing techniques like the Fredman thing you'll read about in the Clayman thread and also have another tool to use when poking around for sounds. An i5 is about 90 to 100 dollars from an online or big box retailer, and it's really not hard to spend a bit of time talking to a Guitar Center audio guy (find a manager who looks bored) about that mic - I got fifteen bucks knocked off of one because the manager offered me a 'special deal' and that place is pretty lenient with that sort of thing - or eBay one. I know Andy also put up clips of a Krank going through a v30 cab and a g12t75 cab, each into a 57 and an i5, so I'll try and track that down so you can hear that difference.
Finally, I babble a lot about taking the transformer out of the SM57 to make it more suitable for metal (especially dropped) guitar - what you do is basically unscrew the thing, take out the XLR connector by playing with the screw near the plug end (that almost sounded sexual...), snip wires (hopefully that didn't...), and boil the body for about 15 minutes (that certainly doesn't...) so you can yank out the yellowish-plastic looking thing (your transformer, responsible for a lot of fucked mids and about 10db of gain) that wires used to be connected to. After that, it's just a matter of reconnecting the wires through the hole that the transformer used to be in and plugging it back in. This'll give you a more room-like sound on tape, because the cheap transformer just doesn't sound good at all, and will help you learn where the fun parts of the speakers are because you're spending less time trying to ignore shit you hate to hear around 4-6kHz and wondering where your lovely bass went.
EDIT: Speaker and microphone comparisons are at
Index of /media/speakers
Jeff
