Sonus,
I've recapped my UREI 813s (the first generation) with Solen caps. There are a number of suppliers in Europe who supply these - I find references to this site:
http://www.tube-town.net/ttstore/ind...olen-Fast.html.
An alternative would be Bennic caps. In most cases, you'll find that these are supplied with higher voltage ratings than the originals. This is acceptable but the physical sizes of the larger ones may cause a problem in placing them inside the crossover boxes and still get the cover back on.
With my 813s, I managed to parallel caps to get the right values but had to consider the size of the caps to make sure it would all go back together. Recapping really did add a lot of lost space and air around the high frequencies and reshaped the mid and bass response.
On the recone, I had to deal with the Altec 604s in my version, which also needed to have the magnets recharged - something I don't think you need to be concerned with on the JBLs.
If you intend to keep the 813s for another 10-20 years, though, I'd recommend you recone _if_ the kits are available. I would not trust that they'll be around forever. Same goes for the horn diaphram, IMHO.
Lastly, I'm suspecting that the subwoofer also needs attention. I refoamed mine but I'm concerned about the spider sagging after 20+ years. For me, that's the fun of restoring old gear, though.
Hope that helps.
Cheers,
David