Henry, in my view your "first instinct" is absolutely right. Go with the QS8s for a 4.0 setup as the next step. Although a sub would be desirable, the M80s have good enough bass so that it isn't a crucial matter at this point. As you've learned, although a separate center speaker has certain advantages, a "phantom" center can be quite satisfactory, and doesn't need an immediate change.
On the other hand, there's no substitute for surround speakers and without them you don't really have a home theater. Both 2-channel(using a mode such as DPLII)and 5-channel source material benefit greatly from the surrounds and make the listening experience more enjoyable.
