I believe the DGX upgrade from the 630 to the 650 included the polyphony change to 128. It is considered an educational piano and you can just not use the super beginner features loaded into it. I am surprised you liked the keys on this one vs the P255 but then I have not played either piano. I have played both their predecessors though, the DGX630 and the P155. I would have hoped that Yamaha improved the keys from the P155 on the P255 but maybe not ! The P155 I did not like at all regarding it's action.
On the DGX 650 you can wire to a sound system through the second headphone jack, you can get all the volume you could ever ask for then, totally based according to the grade of system you wire to ! Of course that can be said for any of your choices just as well. I get the impression that the 650 has decent tone though. Actually the older 630 was not so bad in tone. I would think there would be enough volume up close for say in a dorm room. Adding external sound changes everything, with the right system you can rock the house walls if you wanted to.
I bought my keyboard with no speakers built in knowing I wouldn't be using them but would be using headphones or external sound. I have no use for the weak on board speakers built into keyboards. It had to have enough volume, depth and tone to reasonably keep up with my grand piano at least.
Sir, I noticed little to no difference between the p155/p255. Or maybe I only touched the p255, can't remember. I was not impressed at all. The p35 had very cheap keys, the action itself might have been decent.
Surprisingly the ydp was very very good

.
The dgx 650 felt closest to my acoustic, that's why I liked it. The keys were heavy enough. Really the only downside was the loudness of the audio. Oh and perhaps maybe the gradient between soft touch and hard touch. On the acoustic, if you play softly, you get many types of softness, same with if you play very hard (so press the keys very hard), but with the dgx 650 it felt more like 2 levels, soft and hard. I'm probably explaining it wrongly, but that's what I experienced.
I felt baffled that such a piano was on sale for only 755 dollars. I mean, the p255 costes almost double that and it's not really great in my opinion. I think you experienced the same. Your grand is also on the heavy side in regards to touch correct? If so, then maybe we were experiencing the same.
Funny sidestory: there was this girl, I would say aged 16-18 in the store with her little brother (or it seemed so). She complained about how heavy the touch of the digital pianos was compared to her keyboard. There was only 1 digital there that felt 'heavier' than my acoustic at home, and that's saying something because my acoustic is pretty heavy if I say so myself. The heavy digital was a roland. It wasn't turned on, but the keys were something akin to pushing quicksand away!
I really don't want to regret buying the dgx 650b......It has 88 keys, a lot of bells and whistles, 128 polyfony, good action, close in heavyness to my acoustic.