Car aux is worse than car radio. Any ideas why?

[AC Question]

I drive a 2008 Honda Accord with original stereo. I listen to radio (FM local channels) or my phone music through the car's aux input (in the armrest). I have tried playing my phone music both via spotify and via the phone's mp3 player app. In both cases I've noticed that my aux's sound quality is worse than the radio's! The sound is not bad, but I've to really increase the phone volume (90%) and the car's volume (25 or so, instead of 7 for radio). Any idea why this is happening? Is this because the car stereo cannot handle the phone sound quality, or is there some other setting I need to change (there are not a lot of settings, I've tried modifying SVC - mid/low/high; makes no difference)?
I've noticed this difference esp. in the bass; radio bass, when set high causes the insides of the door to shake, whereas for phone bass makes no difference.

I have high-quality streaming on spotify, and I get excellent sound when I connect my headphones to my phones. So I don't think there's any problem there.

And I don't exactly use cheap aux cable (it's a $2.99 cable, although it shouldn't make a difference).

I don't have bluetooth in the car stereo so I can't compare with that.

 

[weezy2]

Well first off, 90% seems to high for aux input. I would only put the phone to ~3/4 of the volume bar. Anything more than that will cause the sound to peak. Other than that, it sounds like a stereo issue. My stereo volume is slightly higher when I use my aux and/or bluetooth inputs, compared to CD/Radio volume.

 

[Rukbat]

The aux input in the armrest - USB or 3 (or 4) pin plug? If it's USB, it's treating the phone as an MP3 player - probably at 48k, which isn't high enough for music. If it's a coaxial plug (tip and rings all around a common center running through the length of the plug) it could be an impedance mismatch. My 2009 and 2010 Hondas work fine, but the inputs are bridging - very high impedance that will accept almost anything, yours could be lower impedance inputs (which would load the phone audio down to the point that the phone isn't putting out all the signal it can.

With headphones the situation is entirely different - the headphones are expecting a different level than the car stereo.

Also, some phones automatically lower the volume if there's a load on the jack to protect your hearing. (Trust me - from bitter experience - one note that's high enough level and at the right frequency, even if it's only 1/4 second long - can destroy your hearing. The good news is that learning to read lips to some extent seems to be inborn in humans.)

As far as the cable, at the current we're talking about here, unless it's around 100 feet long or more, the "quality" of the cable is irrelevant.