So, you don't put it down to perhaps tweaked EQ and compressor settings for the Parts? Perhaps reworked parameter settings for the Voices?

I have a sneaky feeling that, if Yamaha HAD put a totally new sound engine in there, they wouldn't have missed the opportunity to trumpet it in their advertising...

It is often remarked here at SZ that simply different overall EQ and compressor settings can make a night and day difference to the SAME arranger. Perhaps the perceived difference between the 900 and the 910 might be attributable to merely this (before we go off half cocked about internal changes that may or may not exist)..?