Bass, volume, and fullness of sounds at higher volumes.
Most bluetooth speakers automatically equalize the audio to have less and less bass as audio volume is tuned up in order to provide louder audio, less distortion, and generally a more pleasant experience. These speakers keep the bass and fullness going till almost at the top volume. Additionally, the bass extension is much deeper. This thing probably is clean and loud down to about 60hz or so, making most kick drums and bass really audible and not locking. Most bluetooth speakers roll off at moderate volumes well before 100hz-150hz or so.
Audio isn't perfect, and there are a couple of holes in the frequency response in what sounds like the 100hz region, but a very pleasant audio experience.
Note that this unit is somewhat large, and not easy to transport due to it's shape. Weighs quite a bit as well. When it is plugged into a power source to charge/play, audio output is MUCH better, as power to the 4 drivers inside is 15watts x 4 driver instead 7.5 watts x 4 drivers (as listed by HK).
Negatives: The stereo pairing of two units for true stereo audio requires the HK Connect App, and requires a lot of fidgeting to work each time. It is not consistent and can be frustrating to use the app as it sometimes requires several power off/on cycles of the speaker to get it to recognize properly with the app (have found you'll know it's properly recognized when the power level of each speaker is displayed in the app). The Sony XB series is definitely much more intuitive here and can do it all with the buttons, making it more future-proof.
When paired as a stereo pairs, where one unit is the left a speaker and one unit is the right, the audio and stereo separation are quite good.
Couple other notes, there is a power adapter that charges and/or provides power while playing. It is a separate power brick and power cord that plugs into a standard 110v household outlet, and then plugs into the speaker via a barrel plug at 19v DC. There is also a 3.5mm line input jack and another jack and that I haven't used. The speaker grill/cloth material doesn't seem like it'd be the most durable.
Audio occasionally stutters when changing or skipping tracks/songs manually, though if you pay a whole playlist it's hardly noticeable.