when placing the mic, here is something that you may want to try. Plug the guitar chord into the head, put your thumb on the tip, so that it makes the amp buzz. Then, cup your ear with your other hand, stick your head near the speaker and move it around. You will hear a place where the hum becomes bigger and fuller.....and that's where I try to put the mic.
For posting here AND
The tip that may save my hearing yet!