[uncle duncan]

I've found that bright mics bring out the worst in a classical guitar. This could be what you're hearing in your recordings so far. Overdubbing the vocal should help eliminate phase issues, but it won't solve the mic problem. I've had great luck with a modded Oktava 319 LDC and a modded Oktava MK-011, which is a mid sized condensor intended for broadcasters. Both these mics have very flat response, allowing you to hear the tone of the guitar without all the squeaks and fret buzzes distracting your attention.

Perhaps, if you can't trade up to some flatter mics, you could try foam pop filters on them, or move them around, looking for a sweet spot. Your best investment at this point might be a set of isolation headphones, so you can actually hear what your mics are giving you before you record.