Well, I have been doing my best to A/B the Concero vs my Nuforce HDP with Sigma11 PSU and I am slowly finding the differences between them.
First, though, testing DACs against each other is very hard! Having to switch between sources on my amp along with sources in Foobar as quickly as I can is difficult while a song is playing. I have figured out that I need to pick out just a few notes of a song that show me something I am looking for and replay over and over switching DACs every couple of plays.
The differences between the DAC in the HDP and the Concero are not large, but what differences I notice are definite and important to me. Tonally, I find them quite similar, so I haven't noticed much difference in the balance between bass, treble, etc., but I have noticed a difference in soundstage and the treatment of detail in space. The difference doesn't jump out since the cues are in the background, but I have definitely gotten an improvement in an important sound metric for me. The better realism I get due to this pleases me greatly.
I have also answered my own question about how good the Concero could be when it is based on the same core DAC chip as the ODAC. While I have already sold my ODAC, I had compared it extensively to my HDP. While I still think the ODAC is an incredibly good value, it wasn't better than my HDP to my ears, and the Concero is better to my ears than the HDP. I guess the folks at Resonessence know what they are doing.
From a value standpoint the comparison between my HDP and Concero is harder to figure out. While the DAC is better (or at least the USB input is) on the Concero, the HDP is a very competent headphone amp and has preamp functionality to boot. For $450 ($350 used or refurbished) it is still a killer value.
What I haven't done yet is play with filters (just ordered a remote from Amazon) or tried the Concero as a usb-spdif bridge for the HDP. I know that Nuforce released a usb-spdif to increase the USB performance so perhaps some of the gap between the two devices could lessen with a better USB input. Even if it did, I have still had to add a $150-200 linear psu plus a $50-500? usb-sdif bridge to the HDP to get it to that point.
Overall, I am very pleased with the Concero (love the form factor!) I look forward to the release of the Concero HD to see what improvements come by moving up the chain. Thanks to all who have contributed to this thread and helped me along the way to making the purchase.