Changing the power supply FETs isn't likely to have any audible effect. The Z44s can do everything the amp needs them to do. The original transistors were likely 25 amp FETs (RFP25N05s?). The Z44s are 50 amp FETs.
My 600.2 has the 44 when I purchased it. So, reducing the number of transistors from 6 to 4 has no real benefit to some parameters like distortion?
In this case, money saved