The 770ES builds upon the skills of the 670ES with slightly drier bass, and with a fair bit more dynamic headroom. It's also more forward sounding yet not actually brighter or harsh. In some ways it's probably a mite smoother. Wattage is about the same (ie 100 per side into 8 ohms) but clearly with more current/driving power.
Those who heard the combination at Scalford will attest to how well it all worked together.
I'm now considering moving my beloved Super Tablettes on - not because the Q's are better (they're not as seamless and the ProAcs are more certainly more detailed) but the 2010i's are good enough to not feel like they're strangling the system. Also, they do seem to like being worked fairly hard, waking up big style. The ProAcs really need a sweeter amp I think.