I am still messing with my setup by bringing the rear speakers down a little bit and pushing the soundbar volume up instead. Of all the different "modes", I've found Surround to be the worst - the expanded soundstage sounds very artificial and forced. Standard seems the most true to source for me and provides the best channel separation, followed by Adaptive if you do want a slightly wider soundstage.
Having 33% more speakers in the rear boxes and 33% more watts is going to make the rears sound louder, so all else being equal a Q950A rears should be noticeably louder that a Q950T's rears. Which rears btw have always been loud enough for me, and I sit much closer to the bar than to the rears. It was the prior flagship Samsung bar that had low rear volume level, but as I understand it a firmware patch resolved that since.
Surround Mode used to be the best sounding mode on Q950T with music for me, followed by Standard mode. But it no longer does sound the best on that bar, due to changes in firmware in early April 2021 which made the other 9.1.4 modes sound generally better with music. The consensus now is that Adaptive mode is clearly the best with music on Q950T too. So you're confirming the same trend in the Q950A's modes.