Thank you, I've noted all that and will report back to our lead uke player. I can't answer all your questions (I simply don't know the answers without talking to the 2 fellas in the band, who are the experienced ones), but, if it helps you to advise further, I can tell you that we are not using monitors, our amps (2 Laneys and a Roland AC 33) are behind us, and, yes, the feedback is often worse with the use of effects, when it occurs.
The reason I initially asked about a Feedback Buster, and whether anyone has used them, was that although our lead player uses banjo & 2 guitars as well as the Kala, the feedback only occurs when he is playing the uke. Initially he was playing a Kala Mango, and the feedback was worse with that (it doesn't occur with the electro-acoustic guitar, for example); it's better with the Kala Acacia, but just occasionally it haunts us again. I think I'll go to our next gig armed with a rag (thanks for that tip), newspaper (which I discovered from reading an old thread) and a gravy granules tub top (which, according to another old thread, fits the soundhole of most Kalas), in case of dire emergency.
No need to research more. I think I can tell you your issues with what you have - placement.
By the way, in the world of PA, the Roland is a powered cab or speaker, not an amp. Basically, you don't have a PA, so everything I stated is irrelevant to your setup. You're using music instrument cabs as mains, and placing them behind the mics.
Get those cabs in front of the mics, facing the audience, and your feedback is gone. BUT, now you can't hear yourself because, again, you don't have a PA. Get a one, even a simple one, with monitors, and your world will change.
In fact, you may be able to skip the monitors at first if you use the Roland and Laneys, facing you (instead of the audience) as monitors, and let the PA carry the house. Not the best solution. In fact, not even close by any means. BUT, compared to what you have, well, you know what I mean.
Here's my crystal ball for you, IF you keep performing. At one point, for monitors, you'll get some full range speakers and put them on their side, instead of getting true monitors. Don't worry, everyone does it (doesn't mean there's a not a better way). Or, you'll get a pa, and run the speakers behind the mics, but because you have a good eq, you'll be able to notch the feedback (which you can't do right now).
There's a reason why Yamaha makes a Passport (Fender has a similar one too). i wouldn't use it personally, but its a stepping stone.
