Well, not as much about synergy (I only had the opportunity to try it with the Noble 4 and philips tx2) as much as being too big for my childish hands, a bit too aggressive in the highs and not as musical. And I won't comment on the kinda mixed bag of controls. Anyway, C3 is still kicking ass and honestly while I'm a fiio fan my interest has completely shifted to the dx90.
X5 still needed to lean slightly cooler in tonality for me to spend money. There was still a hint of FiiO house sound present though I had a good demo period, at one stage was going to purchase X5 just for my ER4S. In the end felt I needed to lift the volume rather high to find detail retrieval in the upper mids I'm used too. While I think C3 may not beat X5 in refinement (a notable area for me with X5) and resolution the little Colorfly sounds very clean and airy.
Everytime I send my C3 out on loan I rediscover it again. For example, I rushed to the post office to rescue it before 5pm this afternoon just so I could listen with the Dunu DN -2000 tonight.