So following up on my car experience, I sat down today with my 6s and 7 and some Seinnheiser 280 Pro headphones to do a sound comparison with higher-quality audio. The Seinnheisers aren't quite audiophile-level quality, but I know them well, and they are relatively neutral. I equalized the volume (the 7 needed to actually go up one notch further than the 6s) so there was no perceptual difference due to loudness.
The result is not good. In all cases, the 7 had a loss of lower midrange frequencies, and a loss of the 3D soundstage. Overall all the songs I listened to on the 7 sounded muffled and thin, and most of the soundstage was focused in the center. Brittle and a loss of subtle detail. Listening to the 7 started to hurt my ears after a while the same way that listening to low bitrate mp3s used to do.
Here's a few songs I compared. Everything I listened to was 320 mp3/aac or Apple Lossless files. So, again, not audiophile "oh I won't notice" stuff, but average music files most people have in their library.
David Bowie Five Years. On the 6s, I could hear the rattle of the opening snare and its satisfying midrange thump typical of early '70s recording style. On the 7 there was only a hint of rattle, and the thump had all the meat cut out of the bottom. Bowie's voice sounds thin and distant on the 7, blending into the other instruments. The lower frequencies of his voice are gone, same with the growling bassline.
Bjork Like Someone in Love. This was recorded on a rooftop, and in the beginning you can hear kids playing and a big truck driving. The soundstage on the 6s is amazing and I feel like I'm in the middle of that scene, with the garbage truck rumbling through. On the 7 the soundstage is flattened considerably and the lower-end of the truck's growls and hisses are gone. And then there's Bjork's voice, which on the 7 has all the lower-range roundness taken out of it. It just sounds thin.
Led Zeppelin Ramble On. The soundstage of the intro is fantastic on the 6s, you can pick out the locations of individual instruments, but no surprise, on the 7 it sounds very narrow, hazy, and thin.
Nina Simone Why Keep On Breaking My Heart. The power of Nina's voice is lessened on the 7, both in force and texture. When the backup singers come in, her voice starts to blend in on the 7, while on the 6s it remains a singular thing. As with every other singer and instrument, there's a slice of lower-end frequencies that are absent in her voice on the 7. The bass also lacks punch.
Sonic Youth Rats. So much detail lost in this song on the 7, from the punch of Kim Gordon's bass to the crispness of the snare. Then when Thurston's guitar wash comes in, on the 6s it covers the soundstage with subtle textures. On the 7 it's a vague hiss of sound. And when Lee Ranaldo's voice comes in, there's a growl on the 6s which is completely lost on the 7.
I could go on and on with other songs, but the point is, the 7 with the dongle is shit. This is a major step back after the great audio quality on the 6s, and I'd say it's probably the worst the iPhone has ever sounded. I don't know if they purposefully thinned the sound before it even hits the dongle to make it sound better on the earbuds, or if this is a hardware fumble, but the result is that I'm going to have to use a second iPhone when I want to listen to music because the audio quality with the 7 is not acceptable. It remains to be seen if lightning connection headphones also exhibit this problem. The earbuds are just not good enough quality to compare.
I've tried to stay neutral and open-minded on the no headphone jack issue, but after doing these tests I can say Apple really fucked up here. I am not happy.
A computer mag dropped firsthand knowledge too, but let's defend Aplle!!!