It will not tone down your low end, it might soften your shifts a little but that isn't what a stall convertor is for.
Stall is the RPM at which the convertor stops multiplying torque and goes to a 1:1 ratio. They are often used in vehicles with big cams, or small engines that rev high, or something with turbos or centrifugal type blowers to help get you into boost quicker. They are also commonly used on drag vehicles to regulate the launch RPM.
The idea behind the stall convertor is to get the engine up into the power band using the torque multiplication. It will tend to make the vehicle feel a little soggy or unresponsive when you aren't on it, and when you hammer it you will almost instantly go to the stall speed, which on a supercharged raptor most likely means nothing but wheel spin. Think of it like like doing higher RPM starts with a manual transmission, rev it to 2400 or whatever your stall is then drop the clutch. The effect is largely the same.