Many tubed options might work with your amp, but why not an Accuphase preamp?
I have had the A-50V running in my system for almost 4 years now. I use the Aesthetix Calypso with great results in balanced mode. I have also auditioned a number of other tube designs sucessfully, most notably the Conrad Johnson Prem 17LSII and Mactone, but ultimately went with the Aesthetix due to its balanced design and remote, not necessarily on pure sonic value.
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.
I have had my A-50V (driving Eidolon Diamonds) paired with an Aesthetix Calypso for about a year now with great results. I have a DP-75V and run the Calypso with all balanced connections. I also auditioned the CJ Prem 17LSII and Mactone preamps with great synergy. I ultimately chose the Aesthetix due to its balanced operations (with full single ended connectivity) and remote, as opposed to strict sonic considerations.