My understanding is the iPhone 6+ has a pretty good DAC, even lab tested to very good tolerances.
http://www.kenrockwell.com/apple/iphone-6-plus.htm#measurements
It also seemed comparable to the DAC in my Yamaha CD player with the exact same content.
However, having the volume pinned on my headphone jack output may be introducing unwanted distortion, especially if the pinned output is pushing 1V, while the input voltages on my receivers are fractions of this. It makes sense that if the output is too hot it could cause distortion to creep in. Regardless, I'll test this out at home later tonight to see if the phone at 50% volume has a smoother sound.
This makes logical sense vs. going via USB, as the built in DAC should convert the digital phone signal to the correct level for the preamp.