I would say it really depends on application. If you are using these for a speaker internal crossover network as a filter for midrange or tweeter, I would say that the 400V caps are likely going to be fine. I have seen caps as low as 250V in a crossover network.
I think the one place you have to be very careful is if this cap is used as a compensation cap in a tube amplifier or preamp. The power supply voltages for tubes get extremely high (sometimes as high as 385V-400V). I would NOT substitute a 400V cap for a 600V in that particular application.
I think the one place you have to be very careful is if this cap is used as a compensation cap in a tube amplifier or preamp. The power supply voltages for tubes get extremely high (sometimes as high as 385V-400V). I would NOT substitute a 400V cap for a 600V in that particular application.