I think you're right: Your audio source is the issue. If the source doesn't offers quality codecs to the DAC, then it doesn't matter how good the DAC is.
As mentioned in the above link, I've tested RPi3B+ with Behringer UMC22, and it sounds good. So if you want to find a good audio source for your DAC, you should compare the provided audio codecs of your audiophile hardware and the DAC.
Normally I would suggest to buy a better graphics card to get better audio codecs, which is impossible for a laptop. However, a modern Intel NUC should also provide good audio codecs.
You should try LE Generic AND Generic-legacy on your laptop, before you buy new hardware. Maybe one of those supports a better audio driver with better audio codecs.