To help answer your question, the following is a quotation from Red Wine Audio, which does modification on certain iPods:
"What you may not know is that that the 4th and 5th generation iPods use a very high-quality internal Wolfson Microelectronics DAC (Wolfson DACs are used in quite a few high-end home audio CD players)."
And:
"Please Note The iMod is only offered for the 4th Generation and 5th/5.5 Generation iPods, as they contain the high-quality internal Wolfson dac chip that makes doing the iMod worthwhile.
We DO NOT and WILL NOT modify any of the other iPods, including the new 6th Generation iPod Classic, iPhone, iPod Touch, or any of the Nanos."
So,there appears to be a difference between iPods; unfortunately, these older models may be difficult to obtain. I can tell you based on my own experience that the quality of the file format matters; Ballan's advice is excellent. I have also had good luck with Etymotic plugs, and Grado headphones work well too as these are easy to drive.
"What you may not know is that that the 4th and 5th generation iPods use a very high-quality internal Wolfson Microelectronics DAC (Wolfson DACs are used in quite a few high-end home audio CD players)."
And:
"Please Note The iMod is only offered for the 4th Generation and 5th/5.5 Generation iPods, as they contain the high-quality internal Wolfson dac chip that makes doing the iMod worthwhile.
We DO NOT and WILL NOT modify any of the other iPods, including the new 6th Generation iPod Classic, iPhone, iPod Touch, or any of the Nanos."
So,there appears to be a difference between iPods; unfortunately, these older models may be difficult to obtain. I can tell you based on my own experience that the quality of the file format matters; Ballan's advice is excellent. I have also had good luck with Etymotic plugs, and Grado headphones work well too as these are easy to drive.