The most significant changes in sound characteristics of IEMs with varying impedance response are caused by the source's output impedance as the.
2 Ohms aren't bad but <1 Ohm is desirable. Not every multi-driver IEM reacts the same, though.
You could also test your SD-2 on a source with low output impedance to perceive how it would sound being connected to such a source.
The latest generation MacBook Air, iPhone 4, Galaxy S4 and above, Galaxy Note 4 and above and HTC One M8 have got a pretty low output resistance.
I don't think that there will be huge differences in high frequency but they should be audible.
Just as an example:
That's the UE T.F10's impedance response:
That's how the UE T.F10 (a pretty critical IEM) reacts on different sources (click to enlarge):
As you can see, the MB Air has got the lowest output resistance, followed by the still very good Beyerdynamic a 200 p (1.1 Ohms, causing a dip in the upper highs of about 1.5dB which is only barely audible in quiet environment), followed by the PS Vita which is about at the level of the iPhone 4S (something near 2 Ohms if I am not mistaking) and followed by the Nexus 7 which is by far the worst of them with a very high impedance of propably 50 Ohms.
The signal path should also be free of coupling capacitors else you get a low frequency dip with low impedance headphones, even if the output resistance is low (Cowon J3, Colorfly, iPod Nano 1G.