hmmm... since this is a versus thread. Will throw my cents in.
In short: DX100 wins it.
Sound Quality signature: Plenty to choose from.
1. Bass with authority = firmware 1.3.7 and 1.4.2
2. Euphoric, Musical, lush airy sig = firmware 1.2.7
DAC : Sabre ES9018. If you've heard it in desktop set-ups, the DX100 is just getting warmed up. waiting for future firmware updates to bring it to the next level. The AK100 wolfson DAC is very nice, but to my ears, is missing quite a few ticks to sounding great. And gapless is already supported on sabre dac, not for wolfson 8740 (only 8741) so not sure how iRiver is going to do that with software updates.
The DX100 sounds like it's a singer that hits all the notes and then some, making the performance great.
The AK100 sounds like a contestant on American Idol, looks great, sound great, bbbbut there's always something missing in the performance.
AMP : Though amp wise, personally the DX100 internal amp is hard to beat. There's not an amp available that can soundly beat the the performance of the DX100 amp. With Vorzuge edging it out just barely. And that's a $500+ amp itself. The only amp that flat out best the DX100 amp i've heard, is the ALO rxMKIII-b, and that's almost the price of a DX100 itself. That said, the DX100 amp can power almost any demanding headphones well enough, iems confirm no sweat.
Display : Prefer the larger screen of the DX100 cos once you have 128GB worth of music, scrolling a small screen can get draggy at times.
Flexibility : Coaxial, Optical, true line-out, 3.5mm and 6.3mm options. Almost whichever way to enjoy your music. the jacks are there. It's just short of a balanced input/output to cover them all.
Size : It is bulky but still more pocket friendly over an AK100 + amp. That said, NO single device can come close to what the DX100, be it SQ, output options, amp, etc. None. To do all that the DX100 does, the "stack" need is BIG. i mean really BIG.
Battery : Battery life is a bummer, but hey, for all the stuff it does, it's still OK. Could be better, but still OK.
The DX100 is the audiophile DAP to beat, given what it can offer.
But commercial and general consumer wise, the AK100 can't be beat. Heck, it's the Barbie Doll of audiophile DAPs. It's pretty, small, sounds really good and has local warranty support. Plug a decent earphone and you're good to go.