I am waiting for the Arrow. Supposed to ship in November... The impedance switch is one of the selling points to me - ER4P to S without an external adapter. I also find that I am in love with crossfeed, hence another reason for the Arrow.
As for scaling with an amp - the ER4's scale quite remarkably! I had a fairly profound experience with this earlier today:
I was listening as ER4P from the headphone out of my iPhone 3GS - not bad, but really thin and bass light (Rush "Moving Pictures" great bass, but not prominent in the mix).
I queued up the same song on my Headroom Ultra Desktop with Desktop Power Supply - way overkill you'd think - and plugged in through the P to S cable. Sure the ER4's are still a bit bass light, especially compared to my D7000's or my departed Westone 3's, but wow - the bass is tight and present, the detail is more defined, the sound stage expanded a bit. In short, they are transformed with the upgrade in source and amp.
I am hoping the Arrow will mimic this result portably. I am also looking for an improved portable source - figures, 6 months ago I sold a perfectly good DIYmod for a bargain, along with the LOD's I had...
If I can get to 90% or 95% of the sound portably, I may rethink my move from IEM and portable to desktop listening, as I don't get to just sit and listen as much as I'd like.
Wow, bit of an off topic ramble there - BAck to topic - Waiting for an Arrow, and boy do the Ety's scale well!