We used to install speakers in walls inside yachts - (inwalls or in-ceilings are essentially the only way speakers can get stuffed into some of these cabins)
What are you looking to accomplish with polyfill? In an enclosure the polyfill would tend to increase the "perceived" volume of the enclosure, thus allowing you to use a smaller than designed enclosure for the same frequency response. It only helps with the low end (as the polyfill only has an effect on those frequencies larger than a dimension of the enclosure), and you shouldn't staple it anywhere, it should fill the space behind the speaker.
Since you are going in-wall, I suspect that you aren't also building an enclosure to go in the wall right? You have typical stud spacing, so essentially you have a massive baffle install going - (i.e. much larger than necessary enclosure.)
So why would you need to make your already larger than necessary enclosure seem even larger?
Another possibility is that you may think poly would help dampen reflections inside the enclosure (wall) and cut down on unintended wall radiation (that is the wall vibrating and having audio come out the other side of the wall, or out of phase sound occurring and causing constructive and destructive interference and ruining the in-room sound). The short answer - poly will do nothing for that. Nothing noticeable at least, so don't waste your time.
My gut tells me to use the 1911, but my eyes and brain say the extra ins/outs of the 891 trump the different amplifier.
We used to use concrete and epoxy inside the walls to "build" a sort of enclosure that came "close" to a designed enclosure that fit the crossover curve. It worked well enough to help dampen the higher frequencies, but the low end is coming through no matter what you do.
We used to use mass loaded vinyl sheets (often had to remove wall treatments first to apply underneath) and that honestly helped to deaden the room a lot more than any of that dynamat stuff the car folks seem to love.
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