I have moved from AC-9 to AC-9SE for my power amp (W4S ST 500) and appreciate the upgrade.
I liked and still like the AC-9 very much, only the SE version improved things noticeably. Tightened the bass and improved clarity across the spectrum without any drawback that I can detect.
I dare say that everyone will notice the sonic difference, the contentious point remaining only whether the difference is "for the better in their case" and is worth the extra cash.
For me, the sonic difference was in the right direction and the price difference well worth it.
Those with bright(er) systems might say that the AC9 is better, for its relatively looser bass will give the impression of more bass.
YMMV, but this goes without saying...