Thanks, amigos! May I address the two issues separately:

As to the "can't tell the difference between the M80s on their own and the M80 + sub," my experience is the polar opposite. In my (relatively large) room, the first 15" sub made a "night-and-day" difference in bass. Although the M80s would go to 32 Hz, they couldn't energize the room at that frequency. In short, they lacked authority. Adding the sub made the bass sound far more "real." Adding a second sub smoothed the response and made the bass sound even more natural.

I don't listen loudly, so it wasn't an issue of "more" bass. Instead, it was an issue of control and veracity. The subs DEFINITELY add lots to my M80s.

As to the center being "insufficient," I've actually HAD the VP-160 in my system. It wasn't bad, but for some reason the little VP-100 seems to match the voicing of my M80s better in my room. Perhaps it's because I DON'T listen loudly, but dialogue is clearer and the pans during movies seem smoother with the little VP-100. Go figure...

In many rooms, and/or at higher volumes, the bigger center channel might be the way to go, but for my room, the smaller speaker is the better fit.

Boomzilla

My opinion is worth exactly what you're paying for it!