Be sure to place the Mic that is to be at the front, off to one side of the speaker pointed towards the center, not directly in front of the speaker (Unless you've got a mic that is glutten for punishment). The pressure waves coming straight off the cone can be pretty sharp (I'm assuming you are recordind at fairly loud levels). I usually record my guitars (and bass) with 1 mic 12-18 inches from the speaker pointed in towards the center, and 2 in a stereo array (about 3 feet apart) 5-10 feet away (depending on the room). I always send a dry track as well (which can be used for re-amping, or even de-amping if need be). You definitely need to be cautious of phase problems but you should be able to hear phase problems, while setting up and monitoring your placement of the mics. Often small angle adjustments or placement adjustments can cure a phase problem. If not that's why we have the dry track, simply record 1 mic and the dry signal. Then mute the mic track and reamp the dry track, recorded through any number of different mics, any number of times. You can then adjust any phase or imaging problems within your sequencer.
