I think SNR is probably not the best way to think about consumer electronics anyway. Most of these devices have excellent, nearly quantization noise limited DACs that are far more accurate then is actually useful. Particularly so for anything portable (and thus battery powered).
However, while they'll all give you nearly 16 bit limited performance into a line out, they tend to have fairly limited amplifiers (ignoring Apple, Sandisk which are very good). They also have essentially fixed noise floors that are independent of volume. So a more useful approach is to think about them in terms of the impedance and sensitivity that will give good performance. The noise floor puts a limit on sensitivity, since very high sensitivity headphones will produce more acoustic noise, while the finite output impedance limits how low of an impedance can be driven without distortion.
That said, quality has been increasing. Apple and some other companies will now sell you devices with integrated amps that are better then what passed for entry level dedicated headphone amplifiers 5 years ago.
However, while they'll all give you nearly 16 bit limited performance into a line out, they tend to have fairly limited amplifiers (ignoring Apple, Sandisk which are very good). They also have essentially fixed noise floors that are independent of volume. So a more useful approach is to think about them in terms of the impedance and sensitivity that will give good performance. The noise floor puts a limit on sensitivity, since very high sensitivity headphones will produce more acoustic noise, while the finite output impedance limits how low of an impedance can be driven without distortion.
That said, quality has been increasing. Apple and some other companies will now sell you devices with integrated amps that are better then what passed for entry level dedicated headphone amplifiers 5 years ago.