Hi Kevin, and a warm welcome to the forum. It's great to have another Scott Bonnar owner on board!
Never having owned a SB 590, I am not aware of its idiosyncrasies, but would presume it is similar to the other Scotties in operation.
The Diplomat 590 differs slightly from the Model 430 Diplomat in that the 590 does not have a separate cutter clutch....so as you have found, when you squeeze the clutch handle, the slack in the drive belt is taken up and the cylinder reel and the drive roller then work in tandem.
The speed of the machine across the ground (as far as I can tell) would be controlled by (a) the throttle setting and (b) the amount you squeeze the clutch handle. The engine does not need to run flat out to achieve a good cut; just as you don't have to squeeze the clutch handle all the way in, to get it moving correctly.
Hoping this helps,
