For the parafeed cap, you can use as low as 600 volts - we've never seen a problem with that voltage - though I would still prefer an 800v rating on theoretical grounds. The calculated value is 1.25uF, but anything from 0.62 t0 2.7uF is quite reasonable - the performance does not change much over that range.
For the interstage cap, a 400v rating is probably OK with a metalized-film cap, since they are self-healing to some degree. For other types I would be more cautious unless I knew a lot more about the particular cap - I'd use a 600v cap, myself. The rest of the Bottlehead team will likely agree that I am cautious ... :^) The value of 0.1uF gives a bass cut that is down 3dB at 6.3Hz, -1dB at 12.5Hz. If you go below 0.047uF you will likely hear the loss of deep bass, and if you go much larger (say, more than 0.22uF) the recovery from momentary overload will take longer and may become audibly objectionable.
If you are contemplating capacitance values outside those general ranges, I always suggest using decent but affordable caps until you settle on a value that works for you, before investing too much money on fancy boutique cap$. Different values won't cause damage.
