I have also found if I don't point the mic directly at the hole on the body of the guitar, I don't have as many problems with low frequency feedback. I like to point the mic between the hole and the neck of the guitar. That way, I have more EQ flexibility. Most guitar players don't like direct boxes because they like the sound of their own speaker cabinet. I usually use a dynamic mic either draped over the face of the speaker or on a boom stand.
Condensers work better on softer or distant sources, such as voices or stringed instruments.
Playing a harp is a foot-physical task, so make sure the mic cord doesn't get in the way. As always, discretely listen for the sweet spot. A jazz harpist I once toured with moved the whole instrument around onstage, so we used a wireless lavalier. A support brace is located inside the sounding board of the harp, but be extremely careful when mounting the mic clip not to scratch the wood. I use a good quality stage gaffe tape to cover the teeth on the clip for protection.