A better comparison would be the 3A Sigs plus two 2WQ subs.
Here is Richard Vandersteen's response to some similar questions, translated from all caps:
"...the mid-bass driver on the Quatro is faster and much lower in distortion than the 3a Sig's. The enclosure on the Quatro is very inert, allowing a mush blacker background which improves imaging a lot. Other refinements allow the Quatro to outperform the 3a Sig's with subs in every way except in peak power in the deep bass where the 2WQ's have one more driver per side..."
"Your room is square and will be a problem in the bass for the Model Three's. I suggest the Quatro because of their room bass compensation. The performance of either Quatro will be head and shoulders above the 3a Sig's anyway."
I only got about half way through the Q&A, but I think that about covers it.
In the dealers rooms, the Quatros did have a deeper and more realistic soundstage, but I have to admit that aesthetics probably tipped the balance for me. It turned out that the Quatros needed minimal adjustment in my room (a large volume with few parallel surfaces and speakers more than 3' away from the front wall.)
Perhaps I should have tried the 3A Sigs in my room first. My dealer gives full trade-in value if you do so within a year, and other dealers probably have a similar policy. But I'm impatient.
You should also consider that unless your amp has a built-in high-pass filter (6dB/octave, 3dB down at 100Hz; most HT receivers have 12db/octave filters), you'll need to add on the expense of a pair of the Vandersteen high-pass filters. Also, you much choose single-ended or balanced filters, and you can't mix a balanced filter with a single-ended only amp, which reduces flexibility somewhat.
