My take... I've got the QS8's. Regarding your question about hearing the 5 hz difference in range... I seriously doubt it. If you were watching War of the Worlds at 100 db's and were able to switch between the two models on the fly (in a 5.1 or a 7.1 setup), my guess is you wouldn't be able to tell them apart sonically. Since you're concerned about the load on your wallet, I'm sure you would be happy with the QS4's.
The WAF shouldn't be a big deal... the 8's are only slightly larger. I chose them only because of the small price difference (actually smaller when I bought them) and because they matched the M80's in power handling.
