Thanks for the posts from Fernando and Bob bindus. I already have a very good solid state preamplifier -- the Krell KCT. Perhaps the Accuphase might (or might not) constitute an improvement. But that's not the point of my post. I'm interested in trying a tubed preamp (preferably running in balanced mode). Solid state preamps are no problem (if I were to try other solid state units, I'd try a Klyne as well as an Accuphase). But might tubed preamps (such as the Aesthetix Calypso) prove problematic connected to the A-50V? I'd like to try tubes for the midrange "bloom" and other advantages ascribed to tubes.