My best results have been a bright LDC where the neck meets body about 8" out, darker LDC over right shoulder pointing down at bridge, combined with piezo direct (and not "time-aligned"). Keep in mind, however, I have a circa 1976 Ovation, so it's got it's own kinda sound.
The neck LDC and piezo are panned opposite, the over the shoulder panned off center. Makes for a nice little spaciousness. Sums well to mono. Have to get rid of a bunch of lower midrange with the Ovation, however.
I'm looking forward to getting some C4's, and doing some XY and omni stuff ;-).