I thought only the international version of the S3 had the Wolfson, while the US version has the exact same one as the EVO, the one built in to the chipset both phones have. I've seen too many speculative and conflicting reports to be sure on that though.
You can turn off autoplay in PowerAmp (I hate it as well). Menu -> Settings -> Headset, uncheck Resume on Headset Connect.
Another issue I had with PowerAmp was some flakiness with volume control and distortion when getting a notification while playing music. If you have that issue too, go to Menu -> Settings -> Audio -> Advanced and uncheck Direct Volume Control. You'll have to turn the Gain up after doing this to get back to normal volume.
As far as Beats itself goes, it's not too bad IMO. Kinda reminds me of the old school "loudness" button. It works well for some genres of music, not so well for others.