I had a JBL SCS180.6s speaker system and replaced by iQ90's, iQ60, iQ30's and iQ10's. The only piece that I didn't replace was the subwoofer (150 Watts).
I just spotted a Kef Kube-2 for $ 285 and I was wondering if if could be worth replacing the JBL . Any advice?
Sure. It would be worth replacing the JBL. But there are better values at that price point. However, given the quality of your speakers, I would be looking at 12" sub from Internet direct sub manufacturers HSU, SVS or Outlaw Audio for an equivalent experience from a subwoofer.