Ed,
While the lab boys with way more experience than me will hopefully chime in.....here's my thought. Yes it will probably sound better w/ modern poly caps (probably some old electrolytics in there) but I wouldn't go in and start disassembly of the existing network. What I would do, is build a new one. Replace the entire intact factory version. Heck, you might want to keep your new one outside the speaker cab. In this case, I suppose you just leave the factory unit in place, except not connected.
This way you always have a fall back to what you know and love, and you won't be butchering a classic. Plus, you could even switch back and forth (with a little effort) and see if it really matters.
cheers mate.
joe
