A basic technique that can help control feedback when playing harp into a guitar amp is to set ALL the tone controls at "zero"-- completely off or down as far as they will go. Then plug in and play. Then, if you feel you need to, adjust (turn up) one tone control at a time just slightly in very small increments until you get the sound you are after. Forget the conventional wisdom about turning the bass up and all that. Set them ALL at zero to start. And don't be afraid to leave them there if everything sounds ok. After all, the tone stacks are designed for guitar, not harp. And bass frequencies can cause feedback, too.
Try it. You might be pleasantly surprised.
Btw, I always plug into the low gain input of the amp if the channel I'm playing through has 2 inputs. Not everyone does this for various reasons. But, I think that can also help control feedback, too. Some players prefer the tone they get from the high gain input, though.
FWIW.
JP _______________________________________________ Harp-L is sponsored by SPAH, http://www.spah.org Harp-L@xxxxxxxxxx http://harp-l.org/mailman/listinfo/harp-l