This is one of those tough trial-and-error situations my friend. Tunze makes some powerheads with wider nozzles which reduce velocity of the water. The type of sand you use can have a marked effect on your required maintenance level, and can also have a profound impact on your systems biological stability (more important for well-established systems) - so keep this in mind if you are thinking about switching....
My suggestion would first be to do some research on which powerhead(s) are best suited for your tank dimensions, substrate, and (intended?) livestock. Maybe just doing a forum search for "65" and looking at the powerheads being used.... From there, sell or trade in your Tunze and purchase the appropriate powerhead(s).
I would do this before I swapped out my substrate. Unless you are certain the flow your current Tunze is producing is one you and your tank inhabitants will be able to appreciate long-term.
