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.
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.