Joe, here's what the Quatro manual has to say:
"Can a subwoofer or subwoofers be added to a system
with Quatros?
The Quatros contain integral subwoofers and there
would be no benefit or advantage to adding separate sub-
woofers. When the Quatros are used in a home theater
system, program the processor to redirect the Low Fre-
quency Effects (LFE) and subwoofer information to the
front channels where it will be reproduced by the Quatro
subwoofers. Vandersteen 2Wq subwoofers can be added if
more bass is needed to accommodate a large room. Please
contact Vandersteen Audio about using parallel woofers."
Seems a bit contradictory to me: "No benefit" vs. "2Wq subwoofers can be added if more bass is needed."
The 3A manual does recommend a subwoofer for movie effects.
I'm curious to see how low the Quatros, with their four 8" woofers and two 250W amps, go in my 8000+ cubic foot room. With my NHT XdW subwoofer (dual 10" woofers; 500W amp) out in the middle of the room, bass drops off quickly under 50 Hz.