09-27-15: MattnshilpMatt, no, I doubt that the relatively low input impedance of your Burmester preamp had anything to do with your impressions of the D-5000's sonics. In contrast to the capacitively coupled outputs of the Aesthetix DAC, which can be expected to cause a large rise in output impedance in the bottom octave or two, the Allnic has transformer coupled outputs. And consistent with that it is described in its manual as having an output impedance that is a "constant" (as a function of frequency) 150 ohms.
The point about impedance mismatches is a very interesting one and makes me think that the Allnic D5000 may have a similar issue.
That kind of output characteristic shouldn't result in any issues driving the input impedance of your preamp, or, for that matter, driving longish cables.
Of course transformers can have issues of their own, but given the apparent quality of this unit, and the statements made in the manual about Allnic's expertise in designing and manufacturing their own transformers, I suspect that the reason(s) for your less than glowing impressions of it are related to something else.
Best regards,
-- Al