There was a thread somewhere in here where a forum member compared the QS8s to the QS4s and he found the 8's to be better sounding, closer tonally to the majority of the Axiom line up, ie M22, M60, M80s, if I remember correctly. I had the older QS4s and didn't like the sound the different tweeter had, the new QS4 v2's have the same tweeter now so that isn't an issue. If money really isn't a huge concern then I would go wih the QS8's just for the fact they can play louder and if you ever get a larger HT room you may need the surrounds to go louder. If money is the biggest concern then get the QS4s, I am sure you would be very happy with them as well, as they still offer the same diffuse sound the QS8s offer.
