The xxx and Fi subs should have very similar excursion and tinsel leads. They have slap pads as well. The ava's can slap, but it shouldn't be loud, not like what you seem to be hearing.
I don't see any reason not to maximise an amp's output and turn the gain down. If you only need 1000 watts and your amp does 1000 X 2 or 2000 x 1, hook at X1 and keep the gain down so it is 1000x1 bridged. I'm not sure, but I believe it doing 1000 x 1 bridged is easier on it than working hard to do 1000 on 1/2 the amp, 1 channel. You can always lower the gain but wire it up to optimise the amp.
Keep in mind if you do give it more power though, it will bring your excursion back up, so at those frequencies with as much power as it had before the filter, the issue will return. IMHO I think you need to find out what is happening. Are you simply reaching the driver's limits? If so, keep the power down. Is something loose or broken? A speaker wire slapping the inside of the box? Fix the problem, don't just filter power in that region and ignore the problem.