To clarify for those asking: the stereo speakers aren't both placed at the bottom. One of the bottom grilles houses the bottom speaker, the other houses the microphone.
The second speaker is found in the earpiece.
To my vast surprise, the earpiece speaker seems to be amplified and delivers much richer sound. I tested the speakers with L/R music and was surprised to see that the low-end and midrange is actually tied to the earpiece, not the bottom speaker!
That is in dire contrast to what one would think, especially considering the fact that sound balance seems to be shifted towards the bottom firing speaker, something which DXOMark also confirmed in their audio review. But the earpiece just sounds better when it's playing on its own.
The speakers sound their best at around 60-70% volume. That is when you get the most low and midrange with balanced highs. The higher volumes just turn up the highs and drowns the bass and the low volumes don't really have bass.
As has been said previously - adaptive sound is a worthwhile feature to enable. The difference is noticable.
It's a decent set of speakers, and in some scenarios they play better than iPhones - especially at that 60-70% volume. But they shine in such narrow conditions that it's difficult to praise them wider.