Anti Feedback
Now the trouble is over!? Rather not. Feedback can be reduced in various ways. The playing technique plays an important role, but also the used technology. When everything is at a high level, the feedback is no problem.
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 Squeal Killer
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.
Artec Noise Gate
If a noise gate, then why not a cheap one? The Artec also works but distorts at higher levels.
Boss NS2
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.