It takes a long time to choose components, there are all sorts of parameters to take into consideration. I now have the connectors I want to use in front of me, thanks to the speedy delivery from RS Components (as always).
The ones I will be using are from Tyco Electronics. They have a plastic body, not metal which would interfere with the signals. The current rating is 10A, which is plenty and the connections are silver plated, sturdy and large in contact area. The voltage rating is 450V – the information in RS does not say whether that is for dc or ac but it hardly matters as it will be far above my needs.
These connectors will mean that when a customer wants to upgrade from the Level 1 amp which uses a mains PSU to the level above using batteries, it will be easy, quick and simple to disconnect the amps from the PSU and install the Relay Board and battery pack. None of the existing/original purchase is lost so upgrading to the ‘esoteric’ quality level will be very attractive (imho).