Both the X1 and CAL are efficient headphones, the CAL even more so. I suggest at least a portable amp for both, though for gaming purposes, it's not completely necessary. I dunno about the ZXR's output impedance which may alter the balance of the headphones negatively. may not be an issue overall, but it's possible. If you're gonna spend that much for an X1, don't do it a disservice by attaching it to underwhelming sources. The CAL is a safe bet for minimal amping, output impedance mismatch aside.
I just ordered the CAL. I'll look into buying the X1 at a later when when I can afford a proper amp.
I don't think you saw my last question in regards to the PLYR1. Would you recommend that over the CAL for console gaming or should I stick with the cheaper CAL?