I know the FiiO amps are much cheaper than what you're looking into, but for what it's worth I recently went from a FiiO e5 using the headphone out on my Nano 4G to a FiiO e1. From what I understand both amps are supposed to perform similarly, so my guess is that any perceivable difference would be mainly due to using the line out instead of the headphone out. Using my Yuin G2A's, I see quite a good improvement in SQ with the e1. It's not night and day, but it's just like a veil's been lifted a bit. The music just sounds purer.
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