as you can easily see, I am really new here. I only post since I am currently trying to find the answer to exactly the same question. So here is my current take on this (I also PM'd aamefford on the question HiFi m8 vs. PB2/DB2 so I am still waiting on his input). I have not owned any of the devices, so this is just the verdict I personally built up from information I could gather on the webs.
1) DB2/PB2 vs D6: I have messaged Dan from MrSpeakers on what he would recommend on this issue, and he said, soundwise there is no question, that the DB2/PB2 combo is a very clear upgrade to the D6, so if price is not exactly a super urgent thing, I am advised to go with the former as opposed to the latter.
Downside of the DB2/PB2 vs D6 is here, that the battery life of the combo unit is only up to 13 hours (limited by the DB2, I have not yet figured out if one can rig a battery upgrade or something, e. g. by utilizing some cell phone battery, c. f. the samsung galaxy s4 has a massive 2600mAh battery, but the voltage is too low, but something else might do the trick ... ), vs 20 hours for the D6. Also the DB2/PB2 combo is incredibly bulky (imagine that you strap a stack that is 2inch thick and two inch wide to your audio device, this will not fit into a normal pocket->essentially limits you to put this into your backpack?)
(btw. The D6 gives 650mW per channel, not in total, the PB2 gives 2500mW per channel).
2) DB2/PB2 vs HiFi M8: After getting the quick and helpful response from Dan, I stumbled across aamefford's decision to tell his DB2/PB2 stack and instead get a HiFi m8, so I asked Dan on his opinion on the comparison of the two, which is when I was promptly redirected back to aameefford (PM'd him, and waiting on a response, which will be posted if of interest).
Reading this in retrospect made me realize that this sounds very rude. I meant to say, if he does not post his response himself.
My take on the drawbacks of the HiFi m8 is that its battery life is only 6(!!) hours, which is essentially enough for one day, still I see that that would make an inconvenience for me (also here, fudging in a different battery may be possible, but less so, since the ibasso models all have rollable (interchangeable) op amps, so the insides are much more accessible and also are meant to access, the HiFi m8 to my knowledge is not rollable). Also the output of the HiFi m8 is "only" 700mW per channel (careful here, CEntrance quotes total output power, ibasso quotes per channel) which is almost a factor of four less. Also, rollable opamps/lowpasses on the PB2. Also the stock DB2/PB2 option comes a little bit cheaper than the HiFi m8 (not by much, I think for me it is about 40 or 50 bucks, also depending if I overlooked hidden fees on one or another homepage).
On the other hand, the HiFi m8 is a lot less thick, but longer and wider, the over all bulk is more, but it is thinner, which in my case is the limiting factor, than the DB2/PB2 (I was actually surprised right now, calculating that the HiFi m8 actually occupies more volume in space than two portable units of a more usual size).
3) I haven't yet thought about getting an AK100, and I personally am not considering it worthwhile, hence I have no strong opinion on this other than, not for me ... (I intend to use my phone, I think it is a personal choice of having Android+DAC+Amp vs. AK100+Amp, and I will be all ears if someone can motivate that the difference in Sampling ratios does make a huge difference).
4) Balanced vs. Unbalanced: Well, simply put I am way to uneducated to give any statement about this, what I can say, is that PB2/DB2 and HiFi m8 are the only fully balanced devices.
5) Conclusion: I am personally leaning towards getting PB2/DB2. The only drawback on this is the thickness of the setup, but from a person who has not owned any of these devices, this seems the best choice.
Again, I have no personal experience with any of these devices, I am just listing the features and connotating them with my personal view on the distribution of features.
Hope it helps
Sorry for the wall of text ...
Edited by Luser - 9/21/13 at 10:08pm