A couple follow ups:
-- stackables? Sure, you can try that. I almost always use active mults, so that I know what goes out equals what comes in.
-- getting the voltage offset (steps 3 above) may not be easy to do by ear. I was running the signal into Mordax DATA to view the waveforms. I don't have a good workaround to offer if you don't already have an oscilloscope handy. And I'm not totally sure what happens if the offset isn't fully "dialed in". I believe that would lead to passing DC to the outputs; what I'm not sure of is if DC would be filtered by downchain devices (such as audio outs or audio interface).