Yep, it has to be non-polarised, it's going in a crossover (and I think it's a first-order Butterworth crossover but I have not had a few minutes to sketch the circuit yet) so it will be subject to total voltage reversal at all frequencies within the audio spectrum. The reason it needs to be 220microFarad is that the crossover frequency is 160 Hz, and the voltage requirement is because the sub runs to 400 watts RMS and about 1000 watt peak. In fact if I could get higher voltage I'd likely go for it since I can see from the marks of previous soldering that this crossover has been fried before, possible twice before.

There used to be some lovely ones made in Japan (although expensive) but the main licensee closed down in 2004 and the licensor in 2007, and there seems to be no new old stock left.