The line out dock cables can be used with any ipod or iphone; granted the cable was made to work with that specific iPod (cables can work with multiple models too). People use a LoD to get a cleaner signal out of the ipod that, to be frank, just sounds better than the HO of the ipod itself.
As Jensy stated, you can purchase an iMod or send your ipod classic in (only certain versions) to be iModded for you. With an iMod, you're actually bypassing the internal audio amplifier of the ipod; with regular ipods and iphones you're not completely bypassing that amp.
With both however, you're going to get a better sound that's cleaner and more "powerful" than the HO. You have to hook that LoD cable up to a portable amplifier which can range from 20 bucks (e5) to 500 (iQube). The gap in between is very large and the quality of the amps at that price varies greatly as well.
However, it really depends on how resolving your headphones are as well as how good your source files are if making the purchase of the LoD cable and amp would be worthwhile; some IEMs don't really benefit from amplication and alas, you would be wasting your money (IMO of course).
I see that the phones you are getting are the Equation RP-21s; pretty good phones. I had a pair last year and loved them coming from Senn 555s.
If you have any more questions feel free to ask :) I'd be happy to recommend some ampsl/LoDs for you if you want; no problem here :)