Opus#1, Opus#2, Opus#3, Cowon P1, and some Cayin, and Fiio are all very revealing. AK's are very revealing but I would not suggest one for other reasons, price, UI, EQ, etc. Even your R6 or FiiO X5 Gen III would be more revealing. I'm sure others will chime in with their thoughts.
Don't get me wrong, the Cowon D is great but not for critical listening, nor to plug in $1,000+ IEM's and expect to get it all out of your music such as depth, layering, details, etc. My older Cowon P1 outshines the D and it should for example as it's a true Burr Brown DAC with a very clean amp. They were $1000.00+ when first came out but now are super low priced at $499.00.
The Hiby R3 is out and although it has issues, it's likely the best new competitor with the D in portables. How they stand up against each other is not known yet. Not seen a comparison yet.