Unfortunately the XC40 is too new for aftermarket parts in audio selection, but given that the HK is run on a 600 watt system, it should be fairly easy to upgrade a couple of the speakers cheaply, later on.
When I test drove it I moved the fader around and the back speakers faintly came to life only when the setting was rear biased on 8 or 9. When the speaker fader was completely set on the back, it was a little strange, as though the sound was muffled. The base is clearly lagging and the front speakers seem to be tuned more for treble. I just couldn't get a good balance between the front and back speakers.