I have setup the following configuration in my D10:
AD8656 in LR, and the supplied iBasso dummy buffers in the 2 buffer sockets. The AD8656 is also one of my low-voltage favorites, and seems to be a stellar performer in the D10. I can live with this configuration and find it unlikely that other opamps will be a significant sonic improvement.
The D10 soundstage gives me the impression of a subtle crossfeed implementation, and does not give the "between the ears" impression that many headphone amps deliver. I find that the soundstage is rather "in front" with the AD8656. With the ADA4841-2 and buffers, it was a closer soundstage. Imaging with both is quite good. Overall, the sound is somewhat speaker-like rather than typically headphone.
I also tried the D10 with my IE8s and Images. I will have to turn down the bass when using my IE8s. Lows are stronger than in my previous iBasso amps, at least with the IE8s.
I did have to resort to high-gain when listening to my RS-1s, but the amp handles them very well. I could always re-install the buffers, but still prefer the bypassed sound. I am not sure how well this combination could drive something like the Senn. HD650s. Would probably need the AD8656 and the LMH6643 buffers to get the horsepower for them. Naturally, battery life would be shortened.
The sound from this configuration thru the DAC is superb, probably better than any of the other portables I have heard. The sound is very natural, hard to put a label on it. Just sound very realistic and extremely well balanced. The amp section is better than the stock D1. Nothing is lost when inputing thru the DAC and the internal amp. I can't see any big reason to use it as a standalone DAC. There seems to be no shortage of power at all. I am using low gain and about 10 o'clock position on volume control for a very realistic level of sound.
I am, at the moment, using my Sony F1 headphones and they seem to be a very good matchup.
Due to the low supply voltage to the opamp sockets, there are relatively few opamps that will work well in the D10, however the upside is that those that do work are great sounding opamps!
I find that soundstage and imaging seem to be enhanced by using the dummy buffers, as long as the headphones used are relatively efficient.
I will have to say that based on my experience with the D10 so far that iBasso has developed a product that both cosmetically and functionally is right up there with the best portables.
