Sound perception is subjective, and everyone will have a different opinion on it.
Personally, I have Spotify and the HK sound system, and I think it sounds great. Honestly I don't notice a big difference in audio quality between the High and Extreme options, so I just keep it on High for faster buffering and reduced data usage.
I like the HK better than the base sound system, mainly because its bass is a lot thumpier without being distorted. The Logic7 Surround effect is also really nice, and highlights instrumentation that otherwise is hard to notice.
It is worthwhile to play with the equalizer settings though on the HK -- doing so can dramatically change the personality of the sound system. From the other threads floating around, it seems like there's quite some people who were initially unimpressed with the system but then had better opinions once they played with the EQ.