You can also look at Zinks calls, their rabbit calls are basically an insert of one of their duck calls, take the barrel off your duck call, use that like an open reed distress call and you'll see what I'm talking about. Even their howler looks like it is based off the insert of a goose call, especially since it appears to use their goose call guts.
A little ingenuity/creative thought makes a call for one creature into a call for another into a call for another etc...
I am surprised that you are getting elk like sounds while trying to produce the deeper howls though, if I want to create an elk sound on my howlers I pinch the very tip of the reed and slide up, for deeper howls I am doing the reverse.