I always wanted to do something like this but never decided what kind of topology to use :-\ I imagine this like box with unbalanced inputs and transformer balanced outputs with few fixed gain positions. There is a lot of equipment especially synth modules and keyboards with "boring" output stage so this box can add some life to it . Also this can be used as additional line driver for DAW's or soundcards. When I say "life" I don't think about distortion box. I think about some nice "expensive" character I probably forget some nice sounding line driver so please if you notice that vote for "something else" and post the missing one ;D
Cheers
Cheers