[Sahara]

I currently have a stationary system based on the Stax SR-009 and I am fully satisfied. I have recently acquired a SRS-002 set, but for maximum portability  i'm very curious to know something more about the custom IEMs, which I only read good things.
 
I have no experience with them and I have many doubts about how many drivers are the necessary or ideal for a good configuration, sealing capacity, brands and models..
 
Some of my questions are:
What are the main differences between the CIEMS and the IEMs?
How is the request? I do not understand very well if they are personalized by the customer  based of the type of morphology of the ear canal.
 
What are "custom" refered? Drivers? Sealing caapacity?...

Are they difficult to drive? Do I need a dedicated amplifier? I would like to use with an iPhone 4S
 
Are they comparable to a Stax SRS-002 set?
 
Thank you for your attention and help...

 

[Greed]

CIEM = Custom In-Ear Monitors
 
Meaning, the IEMs are molded to your ear, thus making it a custom in-ear monitor. Typically the word custom has no bearing on how many drivers on in the IEM, or sealing capacity. The sealing capacity is pretty standard in customs, and depends mainly on how well your ear impressions go. 
 
I'v heard the Stax SR-003, and they sounded great, but for portability they aren't good at all. They don't isolate noise at all, and they are technically open so they leak as well. 
 
Now, as far as, sound goes... if you have the money to spend, some of the best CIEM are the FitEar series. They are said to be very neutral, almost to point of being dry and analytical. Now, that doesn't necessarily mean they sound bad, it all depends on if that is the sound you are seeking. They are very expensive (IMO), but the many owners are very happy with their purchase. I would check those out, if you want a similar sound to the Stax.
 
Regarding difficulty to drive, typically no. I've seen some as low as 8 ohm, but usually they will be somewhere between 16-50 ohm. Having an amp will benefit with refinement and authority, but they will sound absolutely fine out of a iphone or ipod.