some say using line-out's doesn't give you much ambience, and can make things sound a bit too clinical. often as well, the quality of the line out feed is shite. i had a marshall valvestate once that had a line out so shit that it was better suited as a pen holder. on the other hand, if the line out is good quality, it can be useful for recording. as a general rule try to use as level an e.q. as possible on the amp, don't overdo the gain and turn off any delay, reverb, effects, etc. on the amp. it's better to do this on the computer often, especially in the case of e.q. and reverb. what sounds nice played in a room with a bit of a reverb can often sound a little vacuous once recorded to disk. try mucking about with your sm57. start by hitting record on your computer, and get one person to play the guitar while another person systematically tries different mic positions, recording the positions as accurately as possible as they go. experiment with different mic positions, different distances from the speaker and if you have more than one mic, try different things like micing the back of the speaker (if it's open/accessible). it all adds to the sound.