I'm sure IIzuka can calculate it for you, but i think you shouldn't go much lower than about 5.6µF. 10µF puts the highpass freq at 23Hz so if your can't reach any decent volume under 50Hz anyway it doesn't really matter.
In my cd-player I had 10µF originally and now 6.8µF and I didn't notice any difference, but if I would go much lower I would lose subbase.
I bridged the MKT with a small PiO because that enhances airyness (the very high frequencies will have the quality of the smaller better caps). When you have 2 caps in parallel the frequencies will pass over the particular cap according to their capacity. It is a trick used in crossoverfilters for speakers when large caps are getting to big or really
I'm not really an electronics-buff, but if you have filmcaps (like MKT or MKP), wouldn't that mean that they are bipolar?This
might give you some clues.
And some more reading
And some more on the art of bypassing