DSP6000 and DSP8000 are sealed bass systems while DSP3200-DSP7200 are vented. This has major impact to articulation and extension in the bass regions IMHO.
I think the only real step up on all accounts (bass, mid treble) for a DSP6k is to go DSP8k.
Please can you elaborate on the impact to which you refer.
Where 7k2s differ from the other speakers is that they offer bass alignment. They are very good speakers, but I have not heard them against 6ks.
To move from 6k to 8k is a very expensive step and there is every right to expect a significant improvement. But I have not heard 8ks against 6ks (I have not actually heard 6ks for a number of years but remember them being impressive).
A comparison between 7k2s and 8ks is a different story: yes, the 8ks are significantly better and yes they are significantly dearer.