I have tried an Alesis "feedback eliminator". It was a little box-type thing, similar in size to the Behringer Shark (which I have not tried...but always wanted to...) I ended up returning the Alesis. The problem I had was the "lag time" between the time it "located the feedback frequency" and the time it eliminated the squeal. It worked, but a slight squeal every ten seconds or so was just not gonna cut the mustard for me. That was some time back (1989?), and technology may have caught up with that issue by now. But in any case, the device would still have to go through a "learning period" in each venue until the problem frequencies were identified, filtered and "memorized" by the device. Okay if you have the luxury of a preliminary sound check, but not always doable in a jam or a spontaneous situation.
Since the Kinder AFB+ does not "locate" problem areas, it does not have a lag time. It works on a totally different principle.
I think that the biggest issue with the AFB+ is the price. It is hard to part with $360 for something sight-unseen; or at least thing-untried. Yet, we do it with $1000+ amps all the time. I've said it a hundred times and I'll say it again: The AFB+ is the best piece of gear I have ever bought. Period. I would give up my amp before I gave up my AFB+, even though I paid $900 for my amp. When I go out of town, on vacation, etc., I take my harp case with two mics and my AFB+ in it. If the opportunity ever arises to share the stage or join a jam, I feel assured knowing that I can plug in to whatever amp is available and just wail away. That is something I could never do with a 50lb, "customized-for-harp" amp. That flexibility has no price tag, in my opinion.
In addition, I believe that most tube amps work great for harp with no modifications. The problem has always been that we (harp players) have never been able to get the volume up into the "sweet spot" before feedback occurs. Guitar players get to experience this as a matter of course. You read all the time of guitarists like Page, Clapton, et al going into the studio and plugging into an old tweed Champ with the volume knob pegged. Harp players can't just do that; even with tube swaps. Let alone do that with a Bassman. The AFB+ gives me confidence, total freedom and the ability to choose the right amp for "my tone".
I was so nervous when I sent the $360 away for the AFB+ that you could have sharpened a pencil in my butt. But I am so glad I did it. I can do nothing now but think of all the money I could have saved "experimenting" with so many amps over the years. Once I got the AFB+, I went and bought the amp I WANTED. Before, I was relegated to buying an amp that "worked best". That, too, has no price tag.
John Balding Tallahassee, FL