You already have one of the best air coolers. Is there a reason you're looking to switch? Why do you only have one fan on it now?
Actually I just bought an Aerocool Shark 140mm to replace the Xigmatek, I'll probably move the Xigmatek to the third slot on the HS. So it'll go
[Case Exhaust Xigmatek] <-- [Xigmatek]<--[Heatsink]<-[Aerocool]<--[Heatsink]<--[Intake]