Rega RS5s - side ports in or out?

The manual is spot on to suggest that you experiment.

I have the R9s and settled on the woofers firing out without any adverse effects of one beinng relatively close to both the back wall and side wall. My own experiences mirror what the RS10 manual highlights as considerations as follows:

" ... You can position the speakers with the bass drivers
firing in or out, this is dependent on your personal preference, room size and shape.

As a guide, if the bass drivers are firing outwards this will generally give a larger sound stage, if the bass drivers are firing inwards this will give more precise imaging.

However, there is no written rule so on installation you should experiment with different positions to find the optimum placement that best suits your needs ..."

I have the RS5s and J stereo says it all.
I have the woofers out, as the speakers are on each side of a TV /stereo cabinet.

I have the smaller RS3's and I read somewhere (forget where) to place the woofers in or out based on how far they are from the nearest object or boundary. If the speakers are close to a side wall, then place them in. If the speakers are closer togther but further from a side wall, place them out. I experimented with mine also, but they seem to let you know the best placement because they will become boomy if the woofer if placed too close to a boundary (whether it is a wall, the other speaker, etc.). This probably doesn't help much unfortunately.