The first mic most engineers reach for when recording or running a guitar amp through a sound system is often a Shure SM57. The mic is commonly positioned directly on axis to the speaker/grille, as close as possible.

In many cases, a second mic - ribbons such as the Royer R121 are increasingly popular - is set up a foot or two back, behind the first mic to capture a different perspective on the amp.

Here's another second-mic position to try: use a second SM57, placed as close to coincident with the first SM57 as possible, but angle the second mic 45 degrees from the first mic.

Get the two mics as close to coincident as you can to minimize phase problems, then experiment with panning the two mics in various ways in relation to one another in the stereo field.