Which connection: RCA, XLR, COAX, OPTICAL?

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
 

Thanks! So, the safest bet would be optical?

ethernet-to-S/PDIF converter - something like this?

http://www.ultrahighendreview.com/simple-design-rendu-ethernet-to-spdif-converter/

So, the safest bet would be optical?

No, I didn't say that :-)

While I would definitely rule out USB for a connection of that length, I have no strong convictions about which of the other possibilities to go with.  So I've stated the relevant tradeoffs and concerns that occur to me, and beyond that your guess is probably as good as mine.

Good luck.  Regards,
-- Al
 

ethernet-to-S/PDIF converter - something like this? http://www.ultrahighendreview.com/simple-design-rendu-ethernet-to-spdif-converter/

Yes, that is one such device. Another much less expensive converter from the same company is this one. As I indicated, though, I have no particular familiarity with such products, and I have not taken the time to study them in any detail. For one thing, I don’t know offhand if those particular products are suitable for playback directly from a computer, or from network attached storage, or both.

Another possibility to consider, if the computer is a desktop model (as opposed to a laptop or tablet), would be to upgrade the sound card to a high quality professionally oriented model such as the Lynx E22 ($699), with which you would also need this $40 analog breakout cable, the combination providing analog XLR outputs having exceptionally good drive capability. You would also need an additional XLR cable, such as Mogami Gold Studio which I mentioned previously, to provide the necessary additional cable length. Or, alternatively, it MAY be possible to connect the E22 to the DAC via S/PDIF using this $30 digital breakout cable plus a BNC to RCA digital cable providing the necessary additional length. (I’m not certain of that, however, as the entries in the description relating to S/PDIF are unclear).

There may be other models from Lynx that are also worth looking into.

Lots of possibilities to consider.

Regards,
-- Al