Added Pictures! (2013/11/15)

Thanks so much for this detailed review!  Unfortunate about the OI as I would probably snag it in a heartbeat otherwise... have a JH Roxanne on the way - 15ohms/119db.  May have to wait on impressions with it first.  Also the Herus is on my radar, but going that route I'll hold off to see how the Geek Out fares.
 
Have you had a chance to listen to the Theorem 720?

The output impedance shouldn't be a huge issue, honestly. It'd be nice to have something <1 ohm, but 2 ohm isn't a deal breaker. Quoting impedance at 1 kHz isn't going to tell anyone much about the overall impedance curve of multi-driver IEMs. I would start to worry if the impedance of the Roxanne somehow drops to 5 ohm or so (a little low, even for BA drivers) at any point, but above that, it'd *should* be okay with the Concero HP. Also, depending on the polarity and electrical phase of the response, it'd be difficult to tell how the FR is affect from a lowered damping factor.
 
Here's the real reason why I wouldn't worry about the OI of the Concero HP with the Roxanne, however:
Jerry Harvey has people listen to it with the AK120, which has 2-3 ohm of OI --- basically identical to the Concero HP. If Jerry Harvey is okay with it, then I *assume* you should be?

No idea about the Theorem 720, unfortunately.