Not so long ago I saw a new shop item flying by called the ZPlane Morphing filter from Lab One Recordings. While the ZPlane Morphing filter is inspired by the Red Queen Equalizer it has the added touch to it to morph two filters using a morph rotary. In theory, this is similar to using 2 filters in parallel and use a balance knob in between to balance two different filters. While I have made setups like these many times before using the Mixer 6:2 or Mixer 14:2 there is one device (which is a free rack extension) that can do something similar in this context.
Because I am about to touch the rack extension realm, and the earliest version I have installed at this point in time is Reason 9.5, I will add that all the example files I will be presenting are made with Reason 9.5. While they should work with Reason 6.5 and above, I hereby cannot fact check if this will work properly in older versions of Reason (just adding this as a disclaimer).