You may not have noticed from earlier posts, but I have a D4. My biggest problem with it is that I need to keep changing batteries, and that is a pain IMO.
I'm not sure if you have tried both D2+ and D4 before you made the comment that there is no contest/comparison for their DAC; I have not, but I have compared their amp section before; D4 is definitely better, but not by leaps and bounds.
Madwolf's suggestion to mod the D2+ is quite appealing; the built-in batt would solve my problem of keep replacing batts, and if the sound is anywhere close to D4 for both DAC and Amp I would be very happy.
I do get it, swapping batts is a drag
Good luck in your quest for the holy grail.