Transparent is now saying that their new USB cable can be used over lengths exceeding what is normally the maximum for USB devices. (This is possible because in audio the send-ack protocol for block mode transfers doesn't happen.) It's also not crazy expensive like some Transparent cables.
The was a recent Positive Feedback Online report on a number of USB to SPDIF adapters, several of which were async. (I'll also note that Steve N., who posted above sells one. I don't currently own any of Steve's products, but his products have an excellent reputation. Also note that Wavelength has a async USB to SPDIF converter now, too.)
If I was in your shoes, I think I would try running USB the entire distance from your computer to your other gear. The USB would go into an async converter that was right at the DAC. (The Audiophilleo converter is especially interesting because it is small and light enough to be connected directly to your DAC's SPDIF in, with no cable. But in any case, you could use a very short SPDIF cable.) Then use good quality 1-meter interconnects from the DAC to your preamp or amp.
The was a recent Positive Feedback Online report on a number of USB to SPDIF adapters, several of which were async. (I'll also note that Steve N., who posted above sells one. I don't currently own any of Steve's products, but his products have an excellent reputation. Also note that Wavelength has a async USB to SPDIF converter now, too.)
If I was in your shoes, I think I would try running USB the entire distance from your computer to your other gear. The USB would go into an async converter that was right at the DAC. (The Audiophilleo converter is especially interesting because it is small and light enough to be connected directly to your DAC's SPDIF in, with no cable. But in any case, you could use a very short SPDIF cable.) Then use good quality 1-meter interconnects from the DAC to your preamp or amp.