UPDATE: Problem fixed, new second try

[kapu]

Listening to your mix on headphones [...]

Again, I'm not Reaper user, but if I remember correctly, you can monitor side channel directly with Reapers built-in functions. From the master section, where you can downmix to mono, you can also choose between mid and side signal. Or if not, just downmix to mono, and then flip the phase of right (or left) channel. This of course happens before the encoding when working with the project, so if you want to check encoded files you of course have to (decode and) import them first. ^_^