I realize the OP said he's not referring to the phone speaker but rather to the output from the headphone jack (if I understood correctly). However, I don't think the two issues are necessarily unrelated. I'm going on the assumption that the phone has an audio card and audio driver(s) much like a computer does. If so, couldn't the main problem be with the internal audio card and/or audio drivers? If this is the case, then different models of headphones and/or external speakers could only go so far before hitting the limits of what the audio hardware/software is capable of doing.(?)
Personally I'm having trouble with low volume from the *phone speakers* on my new S4. This is resulting in missed calls & alarms, notifications, reminders. It's becoming a real problem. My sister just got an S5, and she's having low-volume issues as well. I also see a lot of comments on android tech forums about low-volume issues with BOTH the speaker and the headphone jack in the S4 and S5.
People are even resorting to rooting their phones and installing "Volume-Booster" apps. Apparently these apps work for many people, and most/all of them require rooting your phone. To me this suggests the issue may ultimately be with the audio driver (?). Either way, if you root your phone it *voids the warranty*, plus you really need to know what you're doing or you'll completely screw up the operating system. For both of these reasons, installing a volume-booster app isn't something I'm willing to risk doing.
If all of this is true, wouldn't the "simplest" solution be for Samsung and/or Android themselves to develop a new audio driver? (And make it available for current owners)?
If the low-volume issue becomes a deciding factor in whether people choose a Samsung (or any Android) phone ever again, wouldn't it be worth it? Personally, this will Definitely be a consideration when I buy my next phone.
Thanks for "listening"...