I don't know if you've already received your E18 and perhaps already tried it with a C&C BH2 that you already own, but anyway, I tried my E18 with my BH2 and the combo works quite well. My ears are not very sensitive to high fidelity, and I must admit it's sometimes hard for me to discern the improvement in sound quality between the E18 alone or the BH2 alone compared to using no external DAC or amp; however, the improvement in sound quality using the E18 and BH2 together is just barely enough that I can definitely discern an improvement in sound quality over plugging my headphones straight into my MacBook Pro. The two most apparent changes I can discern are the stronger bass (likely due in no small part to the E18's bass boost function) and a more spacious soundstage (probably more thanks to the BH2 rather than the E18). So if you already own a BH2, they'll play together quite nicely. However, I've just recently purchased both new, and as the difference in sound improvement is so subtle, I think I'm actually going to be returning both, as, for my purposes, neither are worth the $106/$160, and the two combined definitely aren't worth the $266 for me. It's a shame, because I feel like I lucked out with my E18 working 100% flawlessly with my T-Mobile HTC One, but I suppose I'll save money in the long-run by not being a meticulously discerning audiophile. :)
I really appreciate your honesty there though the HTC One is a very good sounding phone.