thanks for the input.. I thought the balance act was SS not tube. let me be a little be more specific on the amp that i'm looking for. The top tier amp is somewhere between Zana and SDS-XLR in terms of sound quality and must be able to accept balanced input and outputs. tube amp preferred.
The RSA B-52 is a balanced tube headphone amplifier that is also in the equation. The biggest benefit of this amp vs.the 307A is that it can also be used as a stereo preamplifier. It does sound VERY good, but is softer in the treble and slightly weaker in the deep bass. In other words, it has a more traditional tube amp sound vice the 307A which is more neutral. I also thought that the 307A was a little more detailed.
Unfortunately, I do not think there are many other top-line balanced tube headphone amplifier alternatives.