Fellas, everyone has a right to an opinion. I think the HDVA600 is a superb amplifier (and yes, I do own one even if I don't use possesive pronouns like "my" or "mine", that I leave to our American friends here), just not as good as it could have been. What we need are the reviews, be it positive or negative reviews. At 1600 Euro, it is a bit expensive and the HDVD800 is simply too much at 2000 Euro.
I will agree that it is difficult to make judgements based on short-term experience but there are those that can say quite a lot about something if they are professionally qualified in that regard. So one doesn't always have to, in this case, listen to something to be able to say something constructive. After all, if I am a bridge designer who has never walked across some bridge, I would still be able to say a lot more about that that bridge than someone who walked across it thousand times but knows nothing about bridge design. I am quite sure there are people like that on this forum so one doesn't need to insist on listening first and then commenting. It is ridiculous and completely unfounded.
As for the output impedance, I seriously doubt it is 43 Ohm. Nothing is known about this amplifier and the published specifications are vague at best. The noise that some have reported might be due to the fully balanced circuit design which, while a noble idea, often doesn't work like intended. The hassle is zero rejection to common noise which means the noise is only amplified once it enters the stage. The other, more serious problem, is component matching because, and let's not forget, fully balanced means twice as many components. Those components have to be symmetrically matched and this includes both passive and active components. Anything short of that would nullify the advantage of such a layout and in every other case, differential circuit is the better solution. There is a good reason why best tube amplifiers are never balanced and no serious DIYer / tube enthusiast would ever consider such a topology. One might be smart here and say that we're not talking about tubes here and that's correct, we are not. But tubes, because of their inherent disadvantages and originally intended application, serve as a good example why component matching is critical in a fully balanced design.
What I would be interested in seeing are measurements - real life measurements of the D/A converter stage on line outputs. I'd like to see measurements of the amplifier section as well - IMD numbers, linearity and crosstalk at 20kHz (I suspect it is 80 dB at best).
I have respect for Sennheiser and my experience is that they make things that work and continue to work for years. Even if they could have done a better job with the amplifier, I think it is a worthy addition to their portfolio and I am convinced that future versions (or even revisions) will be better.