Well, I will tell you my experince with my Aphex Aural Exiter. My first "pro system" was a Crest Audio VS2200 Amp, with a couple of SL-36 Cerwin Vega subs, and a DAS Audio PF-215 Speakers. All the system was in full Range. When I turned on the Aphex, the sound was improved a lot!. I love them, the bass was more notorius and deeper. And the speakers never were damaged, it is not a speaker destroyer because is not a subarmonic synth (that downs an octave the lows). The effect is likely adding a little delay to the bass.
A few moths later, I own a Crest Audio CA-12 and an Ashly xover to biamplifing my system. So with the same parameters in the Exiter, the sound was too muddy, the bass havent very good definition, so I change the parameters, and put the HALF of the effect that have before biamplifing.
Later, I sell the speakers and the VS amp, and buy DAS Audio Reference 218SUB and Raference 215 Speakers. Amps: Crest CA-6 for highs, CA-9 for mids and CA-12 for subs. This system really kicks ass. And the Aphex now, I had to have use it in the Big bottom effect at 10-20%. Theres a lot of difference without them, or overload the signal with it. I still use it, and I love it.
That I wanna try to tell you is that depends on your system the sound that you gonna have with an exiter.
About FeedBack, I never had problems with it, only if I put the mics in front of the speakers. If I had to, I had to turn the Exiter off.
Hope this were usefull to U,
DJ Larry Ata