I understand that is what he is trying to say. I am not certain there is any good evidence showing that a different source would make the amps sound different. I think there is a logical flaw in thinking that changing the source would somehow "bring out" differences in the amplifiers.
Although an Audio_GD Ref 7 might be a better sounding DAC than the DAC Magic- or at least a DIFFERENT sounding DAC- it still doesn't alter the fact that the comparison here is between the two amplifiers. If the amplifiers indeed sound different then their sound signatures will show up whether the input is a direct signal from a pair of Neuman mics in a Blumlein pair flown over Orchestra Hall here in Chicago or a cassette. (OK, well, maybe that is too extreme a contrast... but you get my meaning, I hope) And I really doubt that the difference between a Dac Magic and an Audio_GD Ref 7 is all that extreme. For grins I did some analysis on the output of the Dac Magic and, for example, signal at 10 Hz is down less than 0.2 dB. OK, so maybe it doesn't have the same bass sound character as the Ref 7, but I don't think anyone would say that this DAC lacks bass. And since my reviews are about what the amps DO to the bass from the source, and not about the source itself, I fail to see how a different DAC would change the perception of differences of sound between two amps.
While I don't see how it could make a difference in the results of these amp tests, I have an open mind- if someone cares to lend me an Audio_GD Ref 7 for a week I would gladly repeat all my tests with that DAC.
I will be getting an Audio_GD NFB-11 in a month or so, I can try a few tests using that.
People seem to think the ESS Sabre parts inside the NFB-11 make for a pretty good DAC. The Sabre chips use multiple internal DACs the same way the Ref 7 (etc) use multiple DACs to average out conversion error. The Sabre uses 8 DACs, 4 per channel in dual differential just like the Ref 7, except the Sabre benefits from all this being implemented one one silicon die vs. multiple chips in the Ref 7. I would love to do blind A/B testing with the NFB-11 vs the Ref 7, to see if the various golden ears can really hear these "major differences" in sound quality that they claim to hear between what are otherwise all very well made and designed DACs. I suspect a lot of people go by the assumption that if it costs $1,500 more it MUST be better, and so when they listen the psychoacoustic placebo effect serves to confirm this thinking. Only a blind test could tell if listeners are actually able to hear these differences at statistically relevant levels of confidence.