re 20 and pr 40 can sort of be used in place of each other, except that the pr 40 gets too bottomy if it's too close, so you have to back it off the cab, and then it sounds more like an re 20. but there's a special something to the sound of an re 20, and if i ever get off my butt and fix this re 20 that was given to me, i will use it instead...i think 
