Grainger has given an excellent summary of the practical aspects, so I'll try to summarize just the technical ones.
When the preamp is first turned on, and before the tubes heat up and begin conducting, the is exposed to a very high voltage, as much as 350vDC. Therefor it must be rated to withstand this voltage without damage. A higher maximum voltage is never a problem (except for physical size and cost).
The low frequency response depends mostly on the capacitance and the input impedance of the next device in the chain, usually the preamp or integrated amp. The input impedance of preamps and power amps varies greatly. Eros (and Reduction, the current version of Seduction) are specified for 30,000 ohms or greater, at which value a 1uF cap is down 3dB at 5Hz, which is far below any actual musical content, or anything a real-world speaker can produce. A 0.47uF cap would be closer to 10Hz, still very low. At higher impedances the low frequency rolloff happens even lower. So why do I spec such large capacitors?
I have to thank long-time Bottlehead and occasional poster VoltSecond, who first brought this to my attention. The output interconnect is exposed to electromagnetic fields, many of which can impose hum through capacitive coupling. Thus hum is reduced when the impedance of a conductor is low, so a rational criterion is that the impedance of the output capacitor should be no greater than the output impedance of the source at 120Hz, the most common (non-magnetic) hum frequency. The output (source) impedance of Seduction/Reduction and Eros is around 4000 ohms, so doing the numbers we get 0.33uF. That's why Seduction uses a 0.47uF cap. The Seduction cap is a 250v part, so we need a different part anyhow, and the 1uF/630v part was in inventory for the then-current version of the SEX amp - that's how it got into the Eros.
Bottom line - the minimum voltage rating is 400 volts, and the minimum capacitance is 0.47uF. A higher rating ion either variable will not hurt, but it will not buy you any technical (performance or longevity) benefit. That's not to say it won't buy more subtle subjective benefits - if we truly understood those, they would be technical benefits, but they are not there yet.
