As has been written Many times, the wolfson vs cirrus debate is over and moot. You could rock box but the UI is not user friendly(i had my 5.5 iPod rock boxed, liked the sound, not the IU).
The latest iPod classics and iPod touches sound great. iTunes is fine and easy to use these days, more so If you have a macbook etc.
As an owner and possessor by default of iPod 5.5g, iPod touch 4g, iPod nano(given away) iphone 3 g and 4s I can say this with clarity-that they sound fine. I also have a cowon and the iPod touch sonicmaxpro(BBE) app matches the eq on the cowon.
Your focus should be on headphones/IEMs and matching them to the player you choose. Granted 5.5 has a slight roll off which makes it more bassy/less trebly and goes well with the more trebly headphones. The newer iPods seem to match well with headphones with a bit more bass.
With the latest iPod touch you can get many apps such as sonicmaxpro (The cowon BBE), equalizer, EQu, Stereophonic to tailor the sound to your preference, to name a few apps-there are more, some even free.
With flacplayer app you can play FLAC on your iPod touch also.
*portable amps can be a nice addition, but I would spend my money first on a great top of the line headphones /IEMs. I have owned about 5 portable amps and don't use so much these days as the added bulk can be irritating.
Of course there is the CLAS/Fostex hp_p1 - but again added bulk for a slight return and $$$.
In short. iPod touch 64gb----Great headphones/IEM.----and use one the apps.
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@ bowei006 , wow 4,360 posts in 1 year! incredible.