I'm not sure if you are looking to compare the Apeks to Zeagle or are you comparing the Zeagle ZX50D to their Flathead VI. Zeagle use to sell the Apeks line of regs in North America for about 20 years. In 2000 (approx) They opted to build their own product line which is modeled after the Apeks line. (Only the names have been changed to protect the innecent). The Zeagle line up goes like this ZX/Flathead VI, ZX/50D, ZX/DS-IV, Envoy Delux, Envoy. All their regs have good quality performance. I have a Flathead and a 50D as well as a set of DS-IV's. I also have a older set of Apeks( Zeagle era) Tech 50D's. The performance of all these regs is comparable.When Aqualung purchased the Apeks line they had to change the names hence ATX 50 was the older tech 50D. The difference in breathing a ZX 50D to a flathead is marginal in performance provided they are setup correctly. Zeagles little Envoy reg is a surprising performer considering it's cost.