Hello, and welcome to the forum. The QS series speakers are the best surrounds I've ever heard. If you are talking about a home theater setup, IMHO, you couldn't do better than using QS speakers as both side and rear surrounds. I think you would be delighted with them.

On the other hand, yes, any old bookshelf speaker would "do" for rear surrounds. They would be neither as versatile when it comes to placement, nor as effective as the QS series, but, if the budget is tight, they will suffice.

Jack

"People generally quarrel because they cannot argue." - G. K. Chesterton