Having recently purchased a Galaxy S6 Active I wanted to weigh in on this topic. For background, in the past couple of years I have owned several smartphones including iPhone 5s, iPhone 6, HTC M8, Galaxy Note 3.
I posted on another thread last year that the 5s was reference sound quality at least as far as smartphones go. While the M8 has an impressive amplifier and can drive most any set of cans, for pure audio quality the 5s beat everything else I'd owned.
As the Eagles song goes, "there's a new kid in town". And a new king.
The S6 brings a new level of clarity and soundstage to the table. I suppose it's the Wolfson chip but I am not into technical specs. What matters is what I hear. I overwhelmingly prefer the S6 audio (headphones and Bluetooth) over any other portable audio device. I am literally rediscovering my music collection, which is all ripped at either MP3 320kps or AAC 256. And I am not using an external amp because I don't have any high impendence cans.
The external speaker is another story. It doesn't suck but it's nowhere near as good as the recent iPhones, or needless to say the M8. In short, it gets the job done for notifications, speakerphone calls, and the occasional YouTube video but I'd never want to be stuck listening to music, or an entire movie on the speaker. And I can't understand why Samsung insists on the rear placement of the speaker. Lay the phone flat on its back, and the already marginal sound quality takes another hit. Why not put it on the bottom like the S6? There's a reason Apple has had a speaker in that location forever.
In addition to the outstanding audio quality, the S6 Active has a number of great features including: massive battery, waterproof, shockproof (I am using a thin TPU case to protect from scuffs however), amazing camera, and fantastic screen resolution. Unfortunately, touchwiz is still not as fluid and buttery smooth as it should be considering the processor and RAM that's running it. My iPhone 5 is noticeably smoother. I guess you can't have everything.