Dstruct do this:
- Use 112db Redline Monitor. It rocks. Certainly.
(I don't think it so easy to design a very good speakers emulator plugin).
- Set 4 channels in your Master.
- Set the inputs of Redline as 1/2, the outputs to 3/4.
- Route Reaper so the outputs 3/4 go to your headphone amp.
- The on/off button is the bypass/unbypass of the Redline plugin.
Now when you render the 1/2 channels in the master you don't have to worry about disabling the monitor.