They do have a tendency of sounding flat and lifeless, usually quite dull. They're still very impressive for their size though...
Adding a BeoLab 2 will give you more bass, but won't make things any more 'three-dimensional'. BeoLab 8000s will improve things significantly, but as others have said, their sound characteristics are similar to the 6000s in a lot of ways. They are more dynamic, much more 'powerful' and detailed, but still a little soft at times.
The BeoLab 3s should perform the best to what you're asking for. Very dynamic (even though they're smaller than 6000s), massively more detailed than the 8000s, incredible imaging and to my ears, much more balanced and 'natural' than the 6000s.