The Audiophilleo USB-to-SPDIF converter was popular ~ 10 years ago, and it sounded better (sometimes much better) than direct USB inputs of DACs at that time. It can also be used to support USB for older DACs - just stick it into the coax input of any DAC. Denafrips makes newer, higher end digital converters that will convert USB to SPDIF. And I think Matrix makes a more affordable one.
Sure, digital keeps getting better by miles but somehow never really satisfies analog guys like me. I expect some of the old DACs would hold up pretty well in their sound quality, especially if you pursue a more "analog" sound, but will get absolutely trashed in the measurements game. The top Stax and Sonic Frontiers DACs of yesteryear still might be interesting, and I believe Philips Crown DAC chips are still sought after.