I use the Signal Cable Y cable and it's great. Use it with an iRiver 450 Walkman type player.
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Since you're going into a preamp, you should consider a cable that goes from the dock connector of the iPod to your preamp inputs, rather than from the headphone output. That way you're getting a 'line out' signal that hasn't passed through the iPod's internal amplifier.
I don't know if any of the usual high-end cable makers offer one but high-quality versions are available online, like this and this.
I bought a cable from one of those sellers and am very happy with it. The disadvantage of these cables is that you can't run the iPod off external power while it's connected.