I think with an iem as accurate as an etymotic you will be able to hear the differences between a sansa clip and players starting in the $300+ range. The clip is nice, but we've come a long way baby. At least in sound quality, maybe not so much in user interface. So it would be beneficial to hear new DAPs, but only if you can actually buy one eventually. If not, you're good with the clip.
As far as explaining the benefits of a better sounding DAP. You'll hear the differences as better and you'll know they are there. However, I'm gonna go with the fact that better sound brings a better intangible feeling. Like some sort of inner peace that I didnt know was possible from listening on the go. After all, music is a mysterious element of the world that brings on mental and emotional stimulation. Anything that gets me closer to that is justifiable and why I've stuck with the er4s since I started lurking here.
most of the time what you need to tell DAPs apart, isn't an accurate IEM. my sure way to identify sources is to get an IEM with very high sensitivity and listen to the noise floor that will most of the time be louder on one DAP, or even recognizable, like the little buffering noise I have on my sansa clip every 30seconds or so, added to a little background hiss. a louder background hiss on the sony A15, a very very small background hiss on my fiio X1, no background hiss I can hear with my leckerton amp at normal level setting...
once identified, it's a walk in the park to tell which source I'm using, as long as I have a mighty sensitive IEM(the HF5 is a good candidate for that, much more sensitive than any er4).
the other potential clue would be a slight frequency response change in the treble from the low pass filter. but for that you may need an IEM that extends far, and young ears to do a better job. so even though it seems like the first thing one would notice, it's in fact the one thing I have the most trouble doing. also most of my IEMs start rolling off after 10 khz ^_^. it sure doesn't help.
another easy cue with the right IEM would be impedance variation. the bigger the impedance swing on the IEM and the lower it gets in ohm, the bigger the frequency response change when plugged into something like a 4ohm A10 or a 2ohm fiio X1 or a 1ohm sansa clip or 0.6ohm or my O2. with my IE80 I can't say shiit about that particular variable as it's impedance response is flat. with my JH13 on the other hand, if the low end goes quieter, then the source has more impedance and it's relatively easy to notice.
the last cue can be caps at the DAP output(it's a protection, not a plague!!!!). with ludicrously low impedance IEMs, it will result in a sub roll off. and then a higher impedance IEM(in the low end, not just @1khz) doesn't sound as rolled off, then bingo, there are caps. hinted in my sentence, the ideal IEM for devices with caps are those without a high impedance as the subs will not be as affected.
all in all with my little bag of tricks I can tell almost any source from one another and it's really not because of some golden ears or resolving IEM. now with a pair of etykid (high and relatively flat impedance + low sensitivity) good luck telling most DAPs apart from one another. ^_^ that should be the real golden ear challenge because you can't use my easy "cheats" and have to look for actual sound differences, not manufactured ones.
to me nowadays, a good IEMs is one that does NOT exhibit any of those traits. stability in sound is a feature I value more with passing years. for the er4sr, the sensitivity is a good middle ground so it's unlikely that people would notice any obvious hiss on most DAPs. yet it's really easy to drive. and the impedance is also not so low as to create real problems for the source in most cases. it isn't too much of a mess so you only get a tiny bit more upper frequencies on a sony's 4ohm than on the sansa clip, but it's really hard to notice for a cardboard ear like myself. so yeah if someone believes the new er4 has a signature they'll enjoy(and that's really the main point here IMO), then it's an IEM that will work just fine into most sources and that's a good thing.
the er4s with 100ohm is a very safe bet too as long as we can go loud enough, it makes it relatively easy for the source.