It *cannot* just be about 1's and 0's - surely?

[mansr]

44 minutes ago, crenca said:

Perhaps someone with USB physical layer experience will come along and explain how some endpoints are designed with a built in volume adjustment (which would be a DSP/digital layer adjustment) depending on some signal variation that could be the result of cable length (the OP is mentioning a 3M to 1.5 M reduction) or some other factor such as using weirdly specd "audiophile" USB cables from AudioQuest or any other such manufacturer...

I can think of one vaguely plausible explanation along those lines. Suppose the long cable is so poor that it drops down to USB 1.1 full speed while the short one manages to support USB 2.0 high speed. As a result, different OS drivers might get used, or the DAC might be otherwise differently identified. This in turn could lead to different saved volume settings being applied. A lot of ifs and mights, but it's not entirely impossible.

[mansr]

12 minutes ago, gmgraves said:

It could be anything. I know what AQ says is in it; a 72v battery to "bias" the shield, but who knows? After all, as you point out, they pack a decent sounding USB DAC into a package that is about the same size as a "thumb drive" memory stick! It could easily have a circuit that acts to re-clock the digital volume by +2 dB, although I'd hate to think that they would do that. 

A device that intercepts and alters USB audio data while otherwise being completely invisible to the endpoints would not be difficult to create. Although I wouldn't put it past AQ to do that, I still think some other explanation is more likely.

[mansr]

27 minutes ago, gmgraves said:

Unlikely, but, then, even an unlikely explanation is better than none. USB 1.1 is fast enough to pass 96 KHz audio (IIRC), but if he were listening to 24/192 or DSD over USB1.1 (for some reason), I suspect that SOMETHING will happen!

Yes, USB 1.1 supports up to 96/24 stereo. If the source file has a higher resolution, something would have to downsample it, and that something could well introduce a slight attenuation.

[mansr]

31 minutes ago, sandyk said:

Do you have a suitable USB Female to Female adaptor that you could plug a USB memory stick into ?

 You could then save the outputs of the different USB cables and look at the checksums to see if they are still identical. 

That's not how USB works. Not at all.