Anti Feedback

Kinder AFB

Probably the only working unit in this category. It changes the sound a bit, but that's acceptable. I do not know how it works, so I do not want to speculate. I know some folks who have it, but see it rarely in action. Despite the high price, I would just buy this device, because it's got real technology in it, unlike other constructions.

Lone Wolf Harp Shield

Lone Wolf made such a device, too, but it's just a noise gate. Doesn't really do something but automatically reduces the level if you don't play. The volume itself can not be increased, it is just kind of a guitar pedal again.

Update:
drstrange has tried a modification, that could help other owners of this pedal. With a usable result.
See a picture and a video by Lone Wolf.

The manufacturer says, this unit attenuates very hard and should be used only with strong guitar amps with a lot of gain. Now that's an unusual but perhaps ingenious approach. The promotional text is the usual marketing bullshit and the mention of "CarbonComp resistors" and "Sprague capacitors" always acts a little strange to me, they just want to convey the feeling of quality to the people. Because of the strong attenuation the box somehow works on a guitar amplifier, also the tube generates a little asymmetric waveform. You can not use the device on a harp amp and the price is out of the question.

Soul Blower

This pedal is a simple tone control, one can attenuate the treble. The name "Anti-feedback" does not seem justified .... therefore the price is moderate. Thanks to drstrange for photos and information.

If a noise gate, then why not a cheap one? The Artec also works but distorts at higher levels.

And what did Boss? The NS-2 did not work for me. Whether I have operated it wrong, or if it was a defect or the unit just denies the harp - I do not know.