The 250SE is a step in the current, more "modern" (so to speak) direction of ARC sound, like the REF75SE was too, which culminated into the current 160M/S designs, and that are my personal favorites.
I believe if you dig the 610T (and it seems that you do), and you're not quite ready for a change in sound signature, the REF250SE is a safe choice. I'm sorry that I can't address specific things like soundstaging, as my experience with these amps were on different systems.
My client who had 610Ts went to 750s, though. But he does have a pair of D'Agostino amps that he switches in from time to time, to reduce the wear on the 750's tubes. He has found the D'Agostino to be the most tube-like SS amp, so he doesn't miss the 750s so much.