When I got the QS4's, they replaced M3's at the back. The sound envelopment was so much more. It was everywhere. Far more ambient. Then, I bought the QS8's. They sounded better in every way. More open, dispersed more sound, play louder, fuller, cleaner... I think the tweeter is what really makes the difference. The upgrade is worthwhile if you can get it for a good price.