I just checked and my iBasso D4 uses AD8616 as DAC output opamp. It works single supply 2.7V to 5V and is a CMOS rail-to-rail opamp, just like the AD8692.
AD8066 is a FET opamp.
iBasso D4 has dual DACs though.
You should be carefull since clipping can occur at the DAC output when you swap the opamp. I have no idea about the signal that the DAC puts out, safest is to ask Fiio directly.
I took your advice and ordered some AD8656 and ADA4841-2. Both were mentioned in iBasso D1 thread, the latter was recomended on erji.net (I used google translate).
So another day another swap ;)
I've soldered ADA4841-2 as DAC output IC and LMH6643 as headphone amp (as per erji.net and D1 threads). I've listened the DAP for literaly 5 minutes before going back to work, so no opinion as yet, apart from the facts it works. I'm going to leave ADA in place and do some rolling on the headphone output - LMH6643, LME49720, AD8599 and AD8620.
EDIT: after 2 hours of constant listening to my favourites tracks decided to keep the DAP like it is now, with ADA4841-2 and LMH6643. The sound blew my mind, the level of details, clarity and dynamics is just right. No more rolling for now ;)
Edited by docentore - 11/1/13 at 10:55am