The Bechmark is detailed almost sterile presentation while the Altmann is a more relaxed almost analog presentation. The Altmann's soundstaging is superior too
What everyone is alluding to is that Benchmark DAC1 is so clean it sounds "thin" especially in the lower mids (think in the direction of a horn sound - that kind of clarity) and the exact phase between the two channels makes the soundstage smaller - less depth and less width. If you prefer a laid back slightly less in phase relaxed sound then almost any DAC will sound preferable to the DAC1. If you have a test CD with in-phase and out-of-phase signals then you will know what I mean - it is nothing near as dramatic as a test CD ( maybe 2% or 3% versus the 100% difference on a test CD) but you get the idea.
Anyway this is my observation and it seems to be backed up by what everyone else seems to hear.