I would like to experiment.
That's great, so we'll all learn from what you test. I am curious!
I tried different makes of caps, as I describe in the original post #1. I found that lower ESR caps in parallel (LL, "plain foil" electrolytics, polyester or polypropylene) degrade the sound. this is contrary to what Falcon does, and I am still asking why they go that route, as it clearly changes the balance of the crossover to the worse.
I also experimented while I re-capped the crossovers of my Reference 105. In those old KEF crossovers I'd go with normal, but good quality electrolytics in all positions in parallel to the speakers. In Germany, Mundorf "rough foil" caps with a medium voltage do the job. The same is valid for Fischer & Tausche "rough foil" (ATBI).
I believe that F&T actually supply Mundorf – and they also supply Falcon, at least part of their new ALCAP series. The printing on some of those caps is clearly done in the same way as is on the original F&T and also says ATBI.
What concerns caps in series, some can be low ESR. as I wrote, the input cap for the mid section is of particular importance. so I put a LL ("plain foil") Mundorf there. You'd probably fare even better with a 30uF polypropylene, but the size is far to big to mount it on the original board.
so the baseline is: the idea to swap electrolytics for polypropylenes in an indiscriminate way will throw the crossover off balance. The ESR is part of the circuit, and KEF factored it in. There are some threads here which discuss the fact that you can't easily change a vintage crossover for the better.
So I found that the parallel 7uF (6.8uF in my case) needs to be high-quality but not low ESR – F&T ATBI. An equally mid-ESR Audyn cap sounded much worse. You might counter the lowering of ESR when you use a LL or poly cap if you raise the series resistance in this position. But ESR is not resistance... so this will not a perfect mitigation.