The bang is caused by the 8 feet of water in the vertical pipe, whether it is 2' above the valve and 6' below it, or 6'above the valve and 2' below it. The entire column of water acts on the valve regardless of its actual location. You may need a different check valve, but they will all "bang" when they close. Some are just quieter than others. And the valve can be vertical regardless of what Flowtec says. If there is going to be solids in the pipe they will accumulate against the horizontal valve just as readily as a vertical one.