I've been using the HDP-R10 for a few days now. Very impressed. Coming from iMod 5g + 2Stepdance, the increase in SQ is substantial. Indeed, it's the most significant increase I've yet experienced from a source upgrade.
The two biggest differences that stand out are the increase in resolution, and an increase in all proportions of soundstage (particularly depth). Everything is just so much more refined.
Initially, I thought it sounded a little warm... but after a little brain burn in, I realized that it was just used to the iMod, which was a little cold. Still, the mids are lush sweet for sure. I also detected an increase SQ wise when upgrading the firmware to 8.02
@rudi0504: One thing I am unclear on, is your description of the benefits of an external amp vs headphone out. You described the latter as double amping, as oppose to sound coming straight from the DAC. I can't understand how using the HDP-10's internal amp would be any different to using an external amp (unless of course, the external amp was superior). Either way, sound goes from DAC to amp to IEM i.e. using the internal amp is not double amping. Please correct me if I've misunderstood, or am missing something here.
Talking of external amps - and I direct this at anyone with insight / knowledge - I've noticed that Average_Joe has been using the Portaphile 627 with his DX100. The HDP-R10, of course, uses the same OPA627 as the Portaphile. I'm not sure how many OPA627s the HDP-R10 uses, but know that the Portaphile uses three. Would this mean sonic superiority on the Portaphiles part vs the HDP-R10s internal amp? I imagine there are a host of other factors which I'm failing to take into consideration... but ask purely out of curiosity - I have no intentions of getting an external amp.