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So the sensitivity has a lot to to with drive-ability also....
A bit, that IEM in particular has pretty high impedance with low sensitivity and it's just power hungry. Check out this posting:
http://www.head-fi.org/t/17179/please-explain-headphone-sensitivity To reiterate, my understanding is that high sensitivity headphones are capable of drawing more power from a well-paired amp at the same volume level, so they are more picky as a result, which is why you often find that lower sensitivity is more commonplace and better in portable applications that are using lower power devices. Also, the nominal impedance number listed (40Ohm in the case of the BE) is not exactly accurate for every frequency and every power output, and is more or less an average. It can get a bit tricky to figure out as you can see, I still don't fully understand but I'm learning haha, so I hope this all helps a bit.
I imagine that the Clip+ would be able to drive the RE-262 to listenable levels no problem, but it would only have like 3 hours of battery. In that case an amp would not only afford more battery life, but because of it's high impedence the extra power would tighten it up a bit and clean up the sound and have a pretty audiable improvement overall. I suspect a good portable amp could do that as well with a GR07, but I personally can't say how much of an improvement it would make. I unfortunately don't have much experience with portable amps at this time, or with higher impedence cases like the RE-262 for that matter, that is just what I read of it when I was researching IEMs (and I ultimately chose the GR07 at that time). I am working on a CMOY-style amp at the moment though for practice, and with good quality components
so we'll see how it pans out and if there's much benefit. Others may be able to add in their results from their portable rigs, but from personal experience I feel the BE is quite good right out of the Clip+ or any other portable player I've tried. Here is a good article about amping, so that may further answer some questions:
http://www.inearmatters.net/2008/12/to-amp-or-not-to-amp.html
I do eventually want to refine my portable rig, but since I'm focusing on getting the desktop setup squared away I have to put that on the back burner (and I'm undecided on how to proceed anyway so it needs more research). What I'm currently using though is pretty fantastic I must say for what it cost.