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Yes, at least on the 6+ it has improved dramatically. It's very loud and not distorted at max volume.
 
The 6 has a fuller sound and pretty dynamic for a phone. Definitely better than the 5/5s in that regard.
 
I haven't done a scientific side by side, but the 6+ seems to be able to get a lot louder than the 5s, which sounded great to begin with compared to some of the junk out there XD
 
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