growl3rThe HD598 is a fantastic buy at $99. However, it's often sold at $150. The K7XX is basically a K702 Annie/K712. You can't find them cheaper new than about $330. Even used pairs are hard to find under $300. So from a savings perspective, the K7XX is definitely better by at least double.
Now, whether the price difference is worth the cost, that is completely subjective to the user. I own the hd598 and joined this drop. I don't imagine these will replace the HD598 for me. They are my go-to gaming can with exceptional comfort, but get little to no music head time.
KamakaNice set up man. I decided to keep both for the reasons you mentioned. I'm a touch concerned about PRC country of origin quality control on the K7XX