Ditto!
Notice the placement!! If you point the mic at the cone you'll get woofiness and muddy sound. Place it right up to the grill cloth about 2" in from the outside edge of the speaker.
Kinda like this action:
I think that the general consensus is that putting a close mic right in the middle of the cone will produce the brightest possible tone (picks up the most high frequencies), and at the edge of the speaker will produce the bassiest tone (picks up the least high frequencies). I'd think that the closer to the edge you go, the muddier the tone would be, seeing as the low frequencies would be louder relative to the higher frequencies.
But I do agree with your position suggestion, seeing as it provides a good balance between high and low frequencies, at least on my 2x12 combo amp. This will differ from speaker to speaker, from cabinet to cabinet, and probably from room to room as well.
Also, with a 4x12, make sure to mic an exterior part of the speaker and not a part of the speaker that buts up against another speaker in the cabinet. For instance, mic the upper right-hand part of the upper right-hand speaker and not the lower left-hand part of the upper right speaker. This will help eliminate any cross-talk from other speakers in the cabinet, which might contribute towards the muddiness you're experiencing.
Another thing to think about if you're experiencing a muddy end-result is
don't scoop out your mids! Keep at least some of the mids in your guitar tone because they provide a good portion of the definition of the instrument. Similar to the human voice, if you attenuate the midrange frequencies, you'll end up sounding like an adult in the Charlie Brown cartoons.
