I have been doing some extensive listening to the HUB. My feeling is that it makes the HydraVox a better sounding DAC. On what was already a very detailed and resolving DAC, I am not sure how much more resolution the HUB delivers. Where it is really standing out to me is in the soundstage and imaging.
One of the critiques I originally had of this unit was that the central image in particular was shallow and not terribly stable. This perception is gone almost completely. The depth of the soundstage is improved. Based on my notes, I think it is shallower than the Chord TT2 but borders on rock solid.
It feels like this unit is taking some of the burden off of the Vox and everything sounds like it is flowing more freely and easily. This is much closer to the Weiss 501 in terms of performance than just the Vox/Zap on its own. It definitely is a Rockna DAC and has the Rockna sound presentation that I have become accustomed too.
That one caveat is that I do think it is at its very best in a system with a separate preamp and in a system that is a bit more musical.