So, you can have a variable intput, but I'm assuming you designed it for a constant DC output specific to the Telefunken mics it was designed to power(?) If you could then tweek the output to supply other mics on a custom basis, you might have something there.
I love the sound of tube mics. The U47 is still the greatest ever, right?
I wonder, though, in a blind test, if one were to run a good LD FET mic thru a tube pre, how much difference in that good 'ol tube sound would there be?