I just purchased QS8 speakers and was wondering how they would sound if I placed them on a rear wall behind the listening position as opposed to slightly above and beside.

Thanks.

You never know until you try.
Experiment.
Don't worry too much about manufactorer suggested placement, etc.
You can try their suggestions, but in the end, go with what you like to hear, prefer to hear, and what locations are possible for placement.

The QSx are very forgiving of placement.