The problem with classic equalization in order to eliminate feedback frequencies, is that filters are not suficiently narrow. As
a consequence:

i) just as you experience, it's not always effective, because the gain reduction in the offending frequencies isn't strong enough.
ii) the voice sound tends to loose quality, as many relatively large frequency bands close to the feedback frequencies are rejected.

In order to get effective feedback elimination with a minimum loss of quality, it's essencial to have "parametric equalization", thus the freedom to choose the specific offending frequencies, with the narrowest possible rejection band. Precisely what this small box does! Sometimes it's the only way of stoping feedback, when you really can't move your speakers.

José.