Im newbie to this forum, but Im also an ex audio engineer and studio owner, the technique outlined in stans post is correct, but what I did with great success is leave a guitar plugged into the amp, but a rag tied around the neck to stop any string ring and feedback, you listen for the maximum tonal range of buzz, eg how whomphy, toppy it is, as you move the mic around you will hear a HUGE difference in the sound/tone, you listen for the most of everything, once your there, you have the best mic position for that amp, if you want a different tone, start turning knobs.