I have tried the combination - excellent. I even tweeted about it without explaining myself well... bother. I only have the universal JH16Pro at my disposal, but I can tell you that it pairs very very well with the 601. The 601 hisses much less than the Sony (one of my other DAP loves), and has distinctly more sonorous upper bass and low mids than the X does with any exciting earphone such as the CK10, JH13Pro, JH16Pro, etc.. Again, more laid back earphones with less obvious treble such as EM3Pro or SM3 aren't as good of matches, but still sound excellent.
Am I correct in my assumption that my Walkman X is very similar in sound signature to the NWZ-A818 -- with the same bit of hiss? If so, this is just the kind of comparison I've been hoping for -- and it makes me want to get a 601 even more than I already did :)
I wonder if any of you have tried the 601 with JH16s (I just got my JH16s today!) and have impressions about that combination?
The HiFiman HM601 is definately more full bodied, has a bigger soundstage than the Sony NWZ-A818, and has a different sound signature altogether, I will keep both but prefer the HM601. I think I would describe it as "warmer". The treble crispness is still there perhaps even more impactful, and there appears no additonal bass bloating (in fact bass is solid and tight) and the vocal presentation of the HIfiman is superb. There is no hissing at all using my TMA-1 (very efficient supa-aural), however I will take the HM601 for a spin with the RE-Zeo to see if it is still black.