Unless you've used the DX50/90, X3/X5, AKG 100/120, and so on, WITH high quality headphones, then you can't say anything about smartphones in COMPARISON.
Sure I can, in the same capacity that I can say things about headphones in comparison to each other without having used them. Measurements exist, and if they're from the same source and the source uses good control procedures they are all perfectly valid to compare against each other.
Headphones (or speakers) differ wildly from each other. The overwhelming majority have ~ or > 1% THD across 20-20k and nonlinearities of over 3dB (usually much more). Even the HD800, by all measures a great headphone has over a 10dB swing across just 20-20k. The distortion is also about 0.5% (-46dB) across the spectrum.
Compare this to a phone that is good but not the best, say an HTC M8, where nonlinearity is +/- 0.5dB and THD is ~0.005% (-86dB).
Of course one may argue that the added distortion of the amp/dac circuit is audible (even though the human ear only has an active dynamic range of approx. 70-80dB and very-quiet details are typically glazed over) but then you compare it to the competition.
In comparison the DX90 has a THD spec of 0.0015%. Certainly lower, but then the iPhone is just 0.002% full spectrum as independently measured.
I have an iPhone 6 and a 2 or so year old cowon Z2. I bought the Z2 back when I believed it was truly feasible to not have a smartphone. Since I have moved from a dumbphone to a smartphone it has become obsoleted. The iPhone measures as well as an O2 and ODAC, i.e as good as or better than any schiit product up to a Bifrost Uber in all respects except maximum output power and output impedance (3.13ohms).
That result is thoroughly below the audible threshold. Why on earth would you throw away $900 on a DAP for an inaudible difference when you could have used the $900 to basically move from a noble 3 to a K10?
I really care very little about you, but it would do OP a disservice were he to run out and spend hundreds of dollars on a portable player to get him no audible improvement.