The dBX anti-feedback processing in the GoRack is as good as it needs to be.
There are theoretical algorithms that can cancel feedback into the noise floor, such as acoustic echo cancellation in conferencing systems, but they are hard to do for single sided live audio speech reinforcement and currently impossible for live singing/music.
At a minimum, make sure you have a 1/3 octave graphic EQ or 4 band parametric to even up the system frequency response to avoid wide resonance humps. That'll get rid of broad peaks that notch filters can't deal with. Then you can let the dBx fill in the few small narrow peaks that remain. Beyond that you're right on the edge of GBF, so expect some ringing.
Chances are you might be able improve the situation by a few dB using the above.
For better physical rejection of feedback and ambient noise, the real problem isn't the LDC, but your proximity to the microphone(s). When the ambient volume goes up and reinforcement levels need to go up, you are stuck with the same issues that bands with drummers need to deal with. If you can barely hear the person next to you, then neither can the mic. If you turn up the PA to overpower the ambient 'chatter', then that is going to come back into your mics at some point too. When the room is very reverberant, there is no such thing as 'behind' the mic. Your PA is now outputting a huge percentage of ambient noise (making the problem worse), and your mics are now picking up this extra stage wash from the PA and starting to feedback.
The solution is get on top of your mics.
If you need more mics to be able to do that, then so be it. But in that environment with that much ambient sound, the quality from an LDC will be lost. So you may as well buy some appropriate mics where you can get the capsule as close to the vocal and instrument sources as needed. Maybe that will be an LDC, maybe it'll be a dynamic mic like an SM58/OM7/e835 or maybe even a pencil condenser or direct pickups on your instruments.
For your more intimate gigs, the LDC sounds perfect. But, for loud noisy reverberant pubs, there's no way you're going to get a good amplified sound with a bunch of guys around a single mic unless you're swapping spit.
There's no magic bullet here. If the GoRack along with a broad brush room compensating EQ isn't doing the job, your unlikely to even get a 1 or 2dB improvement from any other processing gear. Feedback destroyers are for that last few dB after room EQ has done the main job to get rid of wide resonance peaks. It shouldn't be finding more than half a dozen frequencies to notch. If it does, then you have wider bands that need to be pulled down in your system EQ first.
This is always a difficult problem, and only you know the real sound levels and the difficulties of getting gear positioned and the band set up. But, at the same time you don't want to place unnecessary limitations on solutions to your problem. Try the above, but be prepared to look at more conventional alternatives.
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