If you are happy with the gain matching from each of your sources, that is they are not so disportionate when changing from one to another that its irratating, just leave them flat. Do nothing - they are only there for convenience.
FWIW, if I had to use them at all for matching purposes, I would choose to lower the settings of the majority to the level of the quietest source than the opposite, which I think is sort of the result of doing what Jim has suggested. I'm very sceptical about using any amplifying source between the source output and the line amplifying stage in the pre-amp.
FWIW, if I had to use them at all for matching purposes, I would choose to lower the settings of the majority to the level of the quietest source than the opposite, which I think is sort of the result of doing what Jim has suggested. I'm very sceptical about using any amplifying source between the source output and the line amplifying stage in the pre-amp.