USB cable hype

Axle, Bulk Transfer looks good but I wouldn't mind Asynchronous with resend. As long as all data gets to buffer jitter doesn't matter since buffer removed timing. New timing is recreated with new stable internal clock. The problem still is computer noise coupled thru (to DAC or capacitively to other cables) or radiated by USB cable. By limitation of cable length (5m) computer has to be close, and that is undesired. I moved computer across the room to the different phase outlet and plugged all my audio gear into power conditioner (with filtering). I keep my cables as short as possible. ICs are 0.5m XLR. Music is send as data in packets over wireless. This data does not have timing. It fills large (few seconds) buffer in my Airport Express. Airport Express is only decent (258ps p-p) with the jitter but in combination with jitter supressing Benchmark DAC1 produces sound that is amazingly clean. With other DACs (like NOS), reclocker could be used.

I am using the IFI usb power supply. That alone made a big difference. I then purchased a pure silver dual headed usb cable on Audiogon, (I forget the name but it is located outside the USA).
For the other connections, I purchased short runs of the Pangea pure silver AG usb cables.

It all mattered, and at each step the sound did improve with a purer sound.

I know the Zero's and Ones should all be the same, but these do sound different.

Kijanki, I like the Airport Express implementation. I tried AE with both optical and analog outputs. I'm pretty sure that my Meridian G68 processor uses a FIFO buffer with synchronous clocking. Therefore, the AE jitter is transferred. In your case, the clocking is asynchronous and jitter transfer is zero. But even in your case, AE noise can enter the DAC through the analog (but not the optical) cable.

Getting back to my OP about functionality vs SQ. I just received an email from George at ExaSound. Working with George is a pleasure. ExaSound DACs use error correction. Therefore, if it is functioning, it is optimal SQ. This is how it should be. DACs that don't implement error correction and use isochronous transfer mode, have to live with whatever transmission they receive. This is not how it should be. Hence my OP about functionality vs SQ. There are some gray areas, even for digital.

Noise is an additional factor. If a cable introduces source noise or EMI/RF to the DAC, then SQ could be significantly impacted. However, the difference between a bad and good cable is neither a steep nor expensive order.

Axle, I remember from Stereophile test of AE that jitter on analog (jitter artifacts) were much worse (about 5x) than optical out. USB cable that does not use +5V supply for anything should not have it. Also shielding should be as good as possible.

Optical is a good choice. Locate the source far away from the DAC, use an optical cable, and you have no noise. Use an asynchronous DAC with error correction and you have no jitter and no errors.

No noise, no jitter, no errors! What more could you ask for? Too bad optical is SPDIF. I guess there is one more thing to ask for: optical USB.