Hmm. I'm assuming by dac, you mean USB sound card?
I doubt that would make a huge difference in your sound, unless you're dealing with noise/interference issues. But changing the overall sound, I don't think sound cards or dacs do that, at least not to a large degree. The only times you might get weird changes like that are if sound drivers use their own processing (and some do, take Waves Max Audio for instance).
As for responsiveness, that's going to depend entirely on how sound cards implement MME, Asio etc. My Yamaha Mo XF keyboard, which was asio compatible, worked pretty well as an asio device. But running a screen reader through it gave me slight, but noticeable lag. I'm guessing the MME support on that device isn't so good.
From what i've heard, offloading sound-related tasks to an external device is only going to help for intensive sound-related processing like in a daw? But don't quote me on that. I just remember noticing that if i used an Asio device on my old machine, I could push it harder before the audio broke.
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