I compared to several laptops and there was a noticeable difference in the dynamic range and the bottom end.
To expand on the other posts, I never had an issue with the sub output because nearly every upper-end active speaker I've used to-date has had an RCA through output. Also, my subs have a variable crossover. A DAC that has a sub output would have a good chance of being either a full-range mono output or using a fixed crossover. I'd almost prefer just another set of parallel stereo RCA outputs.
Not a bad suggestion for an updated, more advanced product, though. But you'd have to pay for that upgrade and I'm not sure the target market needs it.