First of all, the 003 includes just the headphones. It is designed for people who already own an electrostatic amp, as you need one in order to run them. The 005 system comes with either the SRM-212 amp or the older SRM-H (i think) amplifier, and is the stationary version of the 001. In terms of sound quality, it is similar to what the 001 can do with an AC adapter (it basically IS the 001 with an AC power supply), and it also costs about the same as the 001 + the Stax AC adapter/step-down transformer. I would suggest that you get the 001 and the AC adapter/transformer, so you have both the versatility to use them on the move, and the extra sound quality when used at home. The 001 is quite competitive with home setups costing much, much more in terms of sound quality.
Speaking of which...
If you're used to the E2c, and if that's your reference, you're in for a serious shock. The 001 is the best sounding portable under $500, it beats out both the ER-4 (which I own now) and the E5c (which I've listened to). It will be a huge improvement on the E2c! I haven't heard custom-molded IEM's, but I would expect it to be a close call. I've ranted on enough about the 001's sound quality that a simple search through the forums will re-iterate what I have said. Basically, expect strong bass, utterly beautiful flowing mids, and a pretty recessed treble. The 001 is very Shure-ish in it's overall tonal balance, so if you like the warm, bassy sound of the Shures, well this is similar, but miles ahead in terms of quality. The interesting thing is, the full-size Stax models aren't like this, they're much more neutral and balanced across the audible spectrum. Being warm and bassy isn't something that's common among electrostats at all, and a rolled-off treble is something that only this model exhibits. It does, however, show you the clarity, transparency, and amazing instrument separation that electrostatic headphones provide. The 001 does soundstaging and imaging much better than any other canalphone that I've ever heard, since the electrostatic drivers give it a huge advantage over the competition.
As far as your source goes, well, the 001 will show it in all it's inadequate glory. You probably will want to upgrade, and you probably will want to move on to the bigger models
Let's put it this way: when I bought the 001, I wanted a good portable without going the Ety/Shure route. I was perfectly satisfied with my HD590's. Well, now I'm out a thousand bucks or so, and I've got a full-size electrostatic rig that I just can't force myself to stop listening to.
It's amazing how quickly luxuries become necessities. Your wallet will hate you for the rest of your life
A few more things:
1) They're not all that versatile for true portability. I use mine as a transportable system. They're not going to replace your e2c's in that department.
2) They're not going to replace your e2c's in isolation either, since they have none. They're actually an open canalphone (!), and with a 1.5 micron thick membrane, they isolate less than most open full-sized phones.
3) They don't ship with an interconnect, so you'll need one. A good one. At first I used a Kimber mini-to-mini which I bought from HeadRoom for $45, but since it's a very rigid, inflexible IC with straight-line mini's, I can't use it and fit the system in my pocket. Use quality flexible IC's with right-angle mini's if you intend to use the system with a small portable player! And don't use junk IC's either, as the system will mercilessly show you the poor quality of your gear. Micro-detail will suffer a lot with poor interconnects, and the treble will be quite a bit more rolled-off too. Stax are the primadonnas of audio equipment, and they will absolutely not tolerate subpar components. The 001 will work fine off a portable, as long as it's a pretty decent portable, but the full-size systems will need quality sources.
4) They'll go loud, and you really can't tell how loud they are playing without spending a lot of time getting used to them. Watch the volume! I had very mild tinnitus when I got them, and, well, I made it worse. I simply didn't realize how loud I was listening to them. I've forced myself to take a break, and the ears have luckily gone back to normal, but be advised: with electrostatics, you will very often find yourself listening a lot louder than you initially suspect. They have absolutely no distortion whatsoever unless you start pushing them well beyond their intended capacities.
5) Don't use the headband, use them as canalphones. The proper position to wear them is with the cable leads pointing downwards and slightly forwards, and with the phones inserted as deep as they would go and then pulled slightly out. Basically, the position in which you get the maximum bass extension is the one they should be in. They won't be too uncomfortable if worn right, but they will irritate the hell out of your ears until you get the hang of putting them in the right position. The head-band is only there to help keep them in when you're doing something physical, like jogging. They're fairly secure in your ears, but not as secure as tri-flanged canalphones.
6) They need a lot of burn in, and are slightly underwhelming at first. They'll be very rolled-off up top and will sound overly liquid and textureless. That goes away with burn-in, and all the detail starts coming out within the first 40 hours or so. I don't suggest leaving them playing, as Stax headphones are designed to operate against the pressure of your ears, and you might end up damaging the membrane if you leave them playing for long when they're not on your head. Let them burn-in naturally, and you will be amazed at the transformation in their sound.
That's basically most of what you should know. The 001 is a very dangerous first step into broke electrostatic head-fi-dom
