Initial settings (of the programs)
If you are using Reaper v4 version, you take my advice from Jürgen Wolf.
"Important is that the input fx holds Hauptwerk and the output fx holds the reverb module.
You should use the channels on the left side, not those on the bottom. There are two different fx-knobs. More tot he left there is the input fx. If you klick on this a window opens. In this window you have to add the Hauptwerk module (or path). Thats all- then the fx knob more to the right gets the reverb module - or any module from reaper such as an EQ with is also useful."