When I had an Ashdown LB30 I checked with them that it was okay to try lower gain valves in the preamp. They said it was, but it would change the character of the amp.
My current amp, a Markbass TTE500, has four valves in it (one each for gain, compressor, eq and colour) and when I asked them they said that I should only use the valves the amp was designed around.
So, check with the maker, as they should know best!