For computer, audiophile quality is not so much important, because CD player is my primary source for listening.
Good! Because implementing a 20 foot or 6 meter computer-to-DAC connection with high quality results is likely to be problematical and/or expensive.
Yes, it's possible that analog RCA connections could work well enough for your purposes. But I'd have some concern about the possibility of ground loop issues (which can be sensitive to cable length), and I'd also have some concern that the (often unspecified) output impedance of the sound card may be too high to drive the capacitance of the cables without significant rolloff of the upper treble and dulling of transients. Using a very low capacitance cable, such as inexpensive BlueJeans LC-1, would minimize that possibility.
Regarding USB, both the USB 2.0 standard and the manual for the DIA-250 specify 5 meters max for a USB connection, and I suspect that lengths that are even somewhat shorter than that may tend to result in significant sonic compromises. And I'd be skeptical of USB hubs or repeaters that I believe are sometimes used to extend those lengths for non-audio purposes.
Given a choice between a 20 foot digital coaxial cable and a 20 foot optical cable, I suspect that more often than not the coaxial cable would be the better choice, although in some cases noise caused by ground loop issues (to which an optical cable would be immune) could tip the balance the other way.
Also, there are some ethernet-to-S/PDIF converters on the market, and I suspect that some of them would provide better results than any of those other choices (with an ethernet cable covering most of the distance, and a S/PDIF cable covering just a short distance from the converter to the DAC). But while I'm not particularly knowledgeable about such devices, I suspect that good ones will tend to be fairly expensive, and they might necessitate using playback software that you wouldn't otherwise prefer.
Regards,
-- Al
