My personal setup for this is a Shure Green Bullet mic with some light compression and a little reverb. If you place it right in the mix it should really scream. EQ settings will be different per setting and with a little work you can get it to sound just right in the mix. Groovy.

I would prefer the Beta 58A to the SM58 - it seems to give a better result, maybe due to the extended frequency response. Other than that, the thing to check is whether the performer does his harping with the mic cupped in his enclosing hands. If so, you need to wind back on the channel input gain (by about 20dB!) and roll off the bottom end significantly to avoid a boom due to proximity effects. As Al said, try some light compression. However, most players like more than just a little reverb. Use a plate setting rather than a hall.

It depends on the sound you are going for, but often times to get that Chicago Blues sound, you might try using the Shure Green Bullet on the harmonica, send that through an amp (perhaps with some reverb added in), and then mic the amp.

green bullet or 57 have worked for me as well. i used a 57 once and put it through an eq with high/low pass on for a more am radio type effect and worked for the music we were trying to record.

steve