Caps on a Powered Sub

I have a integrated amp (Music Hall a15.2) with an inexpensive powered sub on a second system. I want to use the Hi pass speaker wire connections. The sub's crossover obviously isn't the tops in quality. Is there any way I could get some good quality caps and use them on my bookshelf speakers or on the powered sub to get some better sound? I would like to not use the bookshelf spk full range. Thanks.
What are the voltages and UFD of the capacitors?
I did not get any caps yet. I was asking where do I put them? The bookshelves, the powered sub, etc.
Why not both.

Thanks for trying to help but I have to tell you I am very technically challenged on parts and where they go. Are you saying add them to the existing crossover in the powered sub (below) and then attach a cap to the speaker wire on the bookshelves? Once I decide what to do, I do have someone that can help with my technical shortcomings.
You have to examine the existing crossovers to both speakers. You want to "replace" the capacitors with the same UF, "that stands for micro farad, and it will be printed on the capacitor". VDC will also be printed on the capacitor. It's OK to go over the VDC value but never under. The UF must be the same as the cap you are replacing. Better caps will be larger and probably different, but not in UF.
Correction on my last post. Parts Express would be better able to help you than me in regard to how to replace the capacitors. Ask if there is enough room to replace existing caps with "poly" caps
Thank you very much!