you could do this yourself with a simple A/B switch and forget about a theater through pass input
True, but I still want the tube line stage there to make use of my 2-channel analog sources. And I do not want to fuss with disconnecting the IC-pair to the amp everytime I choose the line stage or the pre/pro as a purist might do. All I ask for is this simplistic "toggle function" wrapped in the line stage's implementation. And for days and days or weeks where only movies might be played, the line stage is powered off.