Actually, depending on the microphone, you might get away with plugging straight into a line input. It's not ideal, but it can work. But it would have to have an output seperate from the mic input, which is definitely not the case in the Tube MP.
In fact, my Dual MP is wired in such a way that if you plug a dynamic mic into the XLR and something else into the 1/4", you can actually hear what's going into 1/4" coming out of the microphone itself. That is, they are wired in parallel so that the mic diaghram acts like a speaker for whatever's plugged in the other hole!