Lordathestrings is absolutely right.
When I mic a cab using a dynamic mic like an SM57 I point the mic at the outside edge of the cone. I will then wiggle it about a bit to find the absolute best spot whilst waring headphones. I do play with axis and location relative to the center of the cone and what ever works, works ! I do all this with out any EQ or compression as that can be added later.
If you are using a condenser mic (careful, you can blow the capsule if you turn up too far), finding the sweet spot is not so hard as it is much larger.
As axis will effect sound and you really want to get the best sound you can down on tape or out a PA, a really good tip (I do this when micking anything) is to get some isolation headphones so you can actually here what the mic is picking up with out any room ambiance.
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