Been looking for an answer to this one. Just bought an A90 and thinking of upgrading my DAC, but choosing from M400 and D90 is a pain in the neck. Since my headsets are all bit bright, so I'm leaning towards M400, but some say M400 with A90 is superb combo, and some say M400's high output impedance makes the combo not ideal(guess we saw from Wolf's post). Did you find a proper answer to the question?