The key to a reeded owl call is the back pressure. Personally I usually tune mine for less back pressure than most, and this helps with the volume issue. I haven’t seen the K&H one so I can’t really help with an exact amount to use, but start by closing both hands pretty tightly at the end of the call. Then if you vary the size of the open space and the size of the sound chamber(airspace inside your cupped hands) you should find something that sounds like an owl hoot.
CW