I've written a lot about bass and its importance in your system in these posts. It's a subject I'll probably continue writing about - so important it is to the music.
As my readers know I am always surprised when I find a system without benefit of a subwoofer. There's perhaps one or two loudspeaker systems in the world that don't benefit from a sub and my guess is most people don't own those systems.
So if most great systems have subs (and they should) then why is it there aren't many great subwoofers available? I haven't been studying subwoofers of late but to my knowledge there are very few truly high-end subs out there. High-end to me, by the way, means they are either very well EQ'd or better still, servo operated. What is a servo? It's feedback on the woofer.
The problem with all loudspeakers is they have relatively high distortion and are not flat - woofers being the biggest culprit of them all. Measurement folk turn a blind eye to real performance standards when it comes to loudspeakers and proclaim 5% distortion and flatness of better than 3dB as magnificent. Imagine an amplifier with those specs.
A servo solves both flatness and distortion failings to a pretty impressive degree - at least an order of magnitude when designed correctly. To add a servo to a woofer, one must attach what's called an accelerometer to the woofer and then use that device as an active part of the system making real time corrections based on a comparison with the input signal. Accelerometers range from expensive to very cheap and all have about the same performance. Devices we used when I was designing such things were less than a dollar.
Does adding a servo increase the complexity of designing an active woofer?
Heck yes, but since when did hard have any bearing on doing what's right?