Copper caps for sure, but they're also much more expensive.
That said, these caps are no longer available.
I would consider looking at copper caps from Miflex, Duelund or Jupiter.
Miflex copper caps come in 3 forms:
-250V which are smaller and better used in box speakers, since they dont take up quite as much space.
-600V these caps get big fast, they need to be used in a large cabinet, external crossover or in OB
-600V same sizes as the other 600V version, but the leads are the same side rather than on opposite ends.
They also only reach values around 10uF to 15uF, so are best used for tweeters circuits.
A great compromise tho is using a Sonicap bypassed with a 0.1uFiflex cap.
Hopefully that give you some ideas to consider.
