Robb,

In my 4,000 cubic foot space, I would never replace a single 600 with dual 400s. There are many reasons but the most predominant one is that two 400s cannot reproduce the couch-shaking bass that I demand for movies and the enveloping, transparent bass that I crave for music. Four 400s might give me what I want but just barely.

Now having said that, the 400 "teased" me with its initial tightness or put another way, its ability to reproduce "snap". If the 600 could expand upon that tightness without losing its enveloping, transparent nature, I believe this would be the best sub on the market at perhaps any price. What I struggle with is whether the next version of the 600 should have a 12dB/octave or 6dB/octave roll-off or perhaps a way to vary the slope or at least select between a number of slopes. It's this slope that greatly contributes to the tightness without making the sub sound bloated.

The 400 is a tremendous sub for smaller spaces though.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated