That's a pretty involved question. Lots of things will influence sound quality, audio codecs, audio drivers, (software, which will be determined by your streaming service, or your software on your other devices) and sound card. Also, you're either gonna be streaming music from the internet, lossy, or streaming music over bluetooth from another computer type device, lossy. So, if you like playing your FLAC and other lossless quality music files, you can't do that with a Dot.
I have used the speakers, tho, and the sound quality is really really good. They aren't very loud. But if you're cool with the sound quality you hear on your computer while streaming from youtube, etc... then I think it's going to be really hard to notice a difference.
You can do more research on the hardware, by following this link https://www.allaboutcircuits.com/news/teardown-tuesday-amazon-echo-dot-v2/
Sorry, not super helpful. I would just be concerned about the speakers volume, while controlled by the Dot.