I've never heard the Monsters, but the CKN70s have more mid bass and more extended highs. The sound is more colored and the guitars have more crunch to them. The FXD80s are more forgiving so I guess they would probably be a better all-rounder. The CKN70s sound a little bit thin in comparison because of the aggressive sound signature. The FXD80s have very nice sub bass and depth, but don't have as much impact or punch. Vocals are about equal, but approached very differently. With the JVCs vocals are very throaty and more forward. The ATs vocals are sweet, lush, and more balanced with the rest of the sound spectrum. The soundstage of the FXD80s has slightly more depth and less width, while the CKN70s have more width and slightly less depth. They both have a good amount of each attribute. I prefer the soundstage of the CKN70s though, it's a little more surround sound and engaging. The instrument separation is better as well. While the CKN70s are warm and bright the FXD80s are neutral and smooth.
Both IEMs have plenty of power and don't need an amp or DAC, but definitely benefit from both. The FXD80s seal with less work for me, but it might not be the same for everyone else. The chord can be a little coily with the ATs, but I feel like it could be tugged a lot harder before something goes wrong. Also, if the wiring on the inside of the housing gets disconnected the CKN70s are very easy to open and re-seal. I haven't been able to get the FXD80s open as of yet.
Thanks for the comparison! I've just pressed the buy button for a pair of black CKNs :D I think I'd enjoy the bass and sound stage on the CKNs more.