As has been discussed in this thread, there appears to be a bug in the most recent firmware version of the SP1000M (1.66) by which the sound signature has become unnaturally "dark" and switches back to the "normal" open sound signature as soon as the DAC filter settings are changed.
The problem is that it appears to fall back to the "dark" sound whenever new music is picked (or the device is turned off and on again).
@JasonNYC : Do you have any indication as to whether or not this has been registered as a bug to be fixed with a future firmware update by Astell & Kern?
Many thanks for your help in advance.