As I am delighted with the modest upgrade from 500 series to G series what would be recommended as an 2nd hand upgrade to my DSP6000's?
If you enjoy five channel sound, I would change the DSP5000HC for a DSP5500HC rather than change the DSP6000 speakers.
The DSP7200 has forward firing woofers, whilst the DSP8000 has side firing woofers like your DSP6000s, so you might find a change to DSP7200s very "different" just because of the change of design.
There is
another thread concerning the relative merits of various iterations of 7200s and 8000s.