the only balanced tube dac i've heard was that thing inside the modded UX-1, and that's not a normal dac.... Oh, and the crazy Empirical audio thing, which i didn't think too highly of. in any case, a 5000$ dac just isn't an option
now this is interesting: http://cls.audiogon.com/cgi-bin/cls.pl?dgtlconv&1201405991
if it did sound as good as the 508....