I don't have headphones, a headphone amp, or Roon. But otherwise sure!
The equipment used, Node 2i streamer - all of it's outputs are active, so the same stream is spit out to both the Coax and Optical SPIDF's. I took one to Modius, one to BF2 - and both in turn to my speaker amp. And I can toggle the respective inputs for A/B.
Is it perfectly level matched, of course not, is it in terms of the end result all but identical and closer than the threshold of hearing, yes I am satisfied it is. Just look at the graph, it's mirrored for 99% of it.
So I think we're still detracting back to this incessant need for level matching at the voltage level. I can't do that, but what I can do is a practical test using a calibrated mic in a controlled position, and to that end the levels are indeed functionally the same.
This test has turned up what I believe to be unexpected results in that there are differences in just a few subtle places with respect to the frequency response. It is a variance large enough in size that we can easily ignore the fact the voltage might not be spot on between them, because the variance in those places is clearly greater and would still exist if the levels were matched.
And that is the question at large here, nothing else. Voltage matched blind test is not going to change that, there is a fundamental variance in response here that doesn't seem to fit with the notion that otherwise ruler flat DACs produce the same output.
I believe (as a layman) the explanation could be filtering related with the way it's handling and interpolating the data. I.e it seams to have retained a bit more signal than the Modius has done. And if 2 devices that handle a sine wave the same way, but not music, does this add any credibility to the subjective claims of others?
Certainly the end by-product of these 2 DACs has resulted in a subtle change in audible frequency response, and I think that implies I am indeed hearing the 2 DACs differently. I've presented relevant data (a level matched A/B test wouldn't actually produce any data for that matter, just give me the mother of all golden ear bragging rights no!
), jokes aside I'm not just throwing audiophile terms about here with zero evidence to substantiate. I'm also here to learn so if my assumptions are wrong please correct.