Yea, this should probably go in the portable forum, unless you have a specific question about headphones to mate to the ipod.
The iPod starts to show it's limits when you start to mate it with higher end equipment.
I have a 4g, and with a line-out to a wooaudio1 tube amp to sony 3ks, using apple lossless it lacks clarity, detail, warmth and sound stage when compared to the same setup with a standalone cd player. Granted, it still sounds pretty damn good for a portable player. But when I'm at home, I'm not using it. I only use it on the go and as my work source. Currently using a variety of headphones, with some shure E5s enroute to me.
The ER4Ps will probably not improve very much with an amp, so you should probably get the P to S converter cable, to turn your ER4P into ER4S. Or, you could go with the supermacro up from
www.fixup.net This is a brand new offering from Xin, and part of the switch upgrade package contains a switch that will turn your P into S without extra cables. I haven't tried this amp personally, but I am very close to getting one. There are reviews in the amp forum of this amp, so I recommend checking them out. My own conclusions are that it's one of the best amps, portable or home, you can buy for under 400 dollars (assuming you max it out, even at 250, it appears to still be quite the bargin.)
Basically, it's all about equipment synergy. You can have the best amp and headphones on the planet, but if you're still using an ipod with 192 encoded mp3s, then you're already bottlenecked.
My own *personal* recommendation is to first look at your mp3 collection, and make sure it is of high enough quality that using higher end equipment will even net you gains. Sometimes improving equipment on crappier mp3s sounds worse because you start to notice how bad they really are. But if we can assume your mp3s are all at least 256k + or even better, lossless then I would suggest stick with your current ER4Ps and going with the supermacro amp. Or getting the P to S cable, and finding the amp of your choosing that fits your price range.
If you want to upgrade your phones too, then spend some time in the headphone reviews forum and read just about every canalphone review there is. There is a TON Of usefull reviews in there. Then go spend some time in the amp forum and see what mates to those phones you choose the best.
All in all, for home use, you might be better off looking into stand alone cd players. Or if you want to stick to digital files, look into high end sound cards (some very good ones run as low as 80 dollars) and turn your computer into the source.
Oh, and welcome to head-fi. VERY sorry about your wallet. (Run away!)
