Not sure if you still have the DAC, but the 12AU7 and the Jupiter coupling caps are the big limitations. They dumb down the sound in the analog system because the DAC is a slightly tweaked version, if not identical to the more expensive DACs. I forget which DACs had which "engines". The newer once supposedly have identical digital sections.
Anyway, it's your analog section that is doing harm to the sound, the digital section sounds awesome. The easy part is converting from the awful sounding 12AU7 to a 6SN7. On eBay there's a seller, garage1217, who makes nice adapters which will do the job. You won't be able to install the cover unless you put a hole in it for the 6SN7. An even more impressive improvement comes from bypassing the Jupiter coupling caps with VHAudio's .1uf ODAM caps, and even better results can be had by using the ODAMs, AND the CuTF V-Caps.
What you love will be vastly improved. The veiled and smeared sound will be gone. The best easily obtained tubes, though you will have to hunt for them, are pre 1964 Soviet 6H8Cs. They are a direct substitute for 6SN7s. You can certainly use the many boutique Chinses varieties, most of which are not half bad.