Honstly, if you are serious about improving your system, I'm most certain there are better ways than simply focusing on possibly upgrading your subwoofer.
For instance you use B&W nautius speakers. Which ones?
Regardless of which they are however, you should be setting them to crossover at abot 80hz on your receiver, period. ESPECIALLY SINCE YOU USE A RECEIVER(and I'm very familiar with the B&K line, and have sold it retail), you need to take advantage of the bass management, as your passive system and receiver combo demands it. Also,speparates, or at the very least adding a multi power amp, would be a big improvement after that. The output from the amps in your receier aren't goign to deliver the current a dedicated amp would. You'll notice big improvements in not only dynamics and power, but every other area with this upgrade.
Then, your setup and calibration of the system is of utmost importance as well. For instance, are all your speakers placed where they're coupling to the room well for excellent frequency response?...at the crossover?...is the sub in phase wih all th speakers from where you sit?...hows the levels?...acoustic?...do you sit close or far from your main speakers in relation your ceiling height and wall proximity?...against the back wall or more out in the room?...where exactly are you crossing over your speakers, or are you running em all full range?...How big is your room/acoustic space and is it open to the rest of the house or attached spaces?(sub requirements would vary here)
I guess what I'm saying is that there are many many other considerations and probable compromises that are keeping your systems potential at bay. The sub is only a part of the equation for excellence and success. Infact I find most peoples mediocrity, even with higher end gear and potentially excellent tools, stems from lack of knowledge, improper setup, integration, execussion, and simply too many variables that are not adressed properly for best results (compounded with multi channel systems BTW).
Most simply don't have a clue...they only read the articles and hear a speaker or two. They then go buy what they think will give them killer sound/video and are dissapointed at the results more often than not! There's WAY more to it than gear.
For instance you've got some good tools in the B&W speakers, with a much better than average receiver. Still, I've been down that road enough to understand there's a lot of ways to make gear like that sound very marginal...and only a narrow path towards getting stellar results in any given room/setup! It's all in the know-how...
