Quote Pete Kaine:
...the addition of a dedicated driver down there will result in the mid's having less work to do and should result in a tighter, cleaner sound
That's the obvious assumption, and if done right is ususally true... but there are some serious caveats to be aware of.
First, and as you suggested, a sub is only beneficical if the room is very well treated to control LF bass modes. A few foam panels aren't going to do it.
But equally important, the benefits to the midrange can only be achieved if the satellite speakers are filtered appropriately to remove some of the low bass energy, but in a way that integrates properly with the sub...
... and even more importantly, that the sub produces an extremely small amount of harmonic distortion -- and I mean really small. That's not easy or cheap to achieve, but it is essential.
A cheap subwoofer will generate a lot of harmonic distortion, and that distortion will smear out across the midrange reducing midrange resolution and clarity instead of improvng it.
Subs intended primarily for home theatre style applications are designed to rumble to support the explosions in film soundtracks. Distortion is often considered a positive benefit because it makes thenm seem louder and more dramatic. They also tend to be tuned for maximum efficiency rather than an accurate response, and so often produce 'one note' bass instead of playing proper bass tunes.
Sub woofers intended or high quality music reproduction tend to be expensive (upwards of £1k for a reasonably accurate model), and to be honest, anything less is likely to do more harm than good to the sound produced by your main monitors.
Technical Editor, Sound On Sound