I had a Z750 with a bad controller I bought used. Mine was either off or full throttle I seem to remember? I sent it out for repair and was told to get a Z1000 controller. Maybe something different in it's design?
I think you aren't getting full power to your rails with the Z750 for some reason? Sounds like the Z1000 works fine.
I bought the Z4000 to run conventional MTH trains.