I think ulysses' comments are on the money, especially the part about getting
rid of the pickup signal and ONLY recording from the microphones.

I often record guitars in XY, and it gives me a good, naturalistic recording.
But if you want a bigger-than-life sound, and am not worried about mono
compatibility or radio readiness, try spacing the microphones apart, pointing
one at the fingerboard around the 16th fret, the other one just below the
bridge. They should be parallel to one another, and equidistant from the
guitar; experiment to find what distance works for you. Again this is NOT
natural, but boy, can it sound big. SM81s can do fine with this setup.

By the way, something very important: what kind of guitar are you playing --
make and model?

Peace,
Paul