Short version:
Note 3 not providing proper auxiliary output, leaving the music sounding flat and nowhere near as good as the phone can provide. I'm looking for a fix rather than an amp app, as this just amplifies a poor signal rather than fixing the underlying issue.  Understand this is not Spotify-specific, but felt as this is my primary media player, I should ask the question here as well.

Long version:
I recently purchased a Galaxy Note 3 on contract and, while the overall function of the phone is stellar and a joy to use, I've been having a pretty annoying audio problem which is really taking away a large portion of that joy. Basically, I drive quite a lot, and I always use my phone to play music through my AUX port with a two way stereo cable. On my previous phone this sounded more than adequate, if not excellent, but with the Galaxy Note 3 I'm finding the auxiliary audio to be missing quite a lot - this isn't just a volume issue, there's definitely a bunch of frequencies just not pulling through to my sound system.

I'm 99% certain this isn't just poor audio quality from the phone itself as, about 30 minutes into a 45 minute journey, the music I was listening to exploded into life. It was the best I've ever heard this particular track sound (even on my previous phone), so I know the phone is capable of much better output - I just haven't been able to replicate this fluke in the few days since it happened.

I've tested it fairly extensively and even obtained a replacement handset as I had free tech support for a month as part of my contract, and they diagnosed a faulty audio jack. As the sound isn't intermittent, I don't believe this to be the case. The audio can also be set at an acceptable level on any other device I connect yet, when I connect my Note 3, you can't hear the audio until I raise the phone's headphones volume to full and crank my car volume up another 4 or 5 notches - and even then the audio isn't as 'full' as it should be; the vocals sound distant, there are instruments much, much quieter than they should be and overall the audio feels incomplete.

I've tested 4 different phones now (my original Note 3 purchased 6th January, an HTC1, an iPhone 4S and now my replacement Note 3 obtained 11th January). The differences between the iPhone or HTC1 playing the same track through the same program (Spotify) is like night and day.

I also tested 3 different auxiliary stereo cables with each of the above phones (2 belkin models and a bog standard cable) and all are perfectly fine with the other two phones but lacking on the Note 3.

Lastly, I tested to see if it was a software issue and ripped 5 tracks at CD quality that I had access to on Spotify, playing each track in the native music player followed by its Spotify version - both suffered the same 'incomplete' feel.

Given these tests and the fact that the phone *has* played amazing audio on one occasion so far, I'm at a complete loss as to what the issue could be. My AUX port and cables obviously work fine, and other phones work fine. It's not a software issue as far as media players are concerned and there's no equaliser setting that I have set up on the Note 3.

Thanks so much in advance if anyone can shed light on this, as it's really one of my main uses for my phone as far as day-to-day tasks are concerned, and the audio is bugging me so much that I've started listening to the radio for the first time in about 5 years 😛

Cheers

Matt