If you want to add a 2 ch tube pre-amp before the SM you only need one as the SM has only one 2 ch input. If you want to add it after the SM it would require 2, 2ch pre-amps. The bigger question is: why?Thinking of adding a tube pre-amp to my system. Anyone have any thoughts on if I could get away with just using one stereo amp before the SM? Sure would save me some money.