I have four B&W 801's, two 603's and a B&W center

A sub would be the absolute last thing you would want with a speaker like the 801. I assume that Elevick would have you run the signal from your preamp to the sub's filter and back to your main amp. This is guaranteed to degrade the quality of your signal. Any sub that can match the level an 801 can play is going to be very expensive , very large or both. You don't need one, and I speak as someone who has 4 pairs of subs and who has used 801s. His statement about power rating is misleading. It would be accurate if it were applied to amps THAT WERE ABSOLUTELY IDENTICAL IN EVERY RESPECT except power. This is never true. A voltage source amp will double its power when you halve the impedance and can drive difficult loads. My M3 which is rated at 275 watts at 8 ohms will do better than 800 at two ohms. Other amps will have reduced power at low impedances. The amps I previously mentioned will have all the power you need, more than the 500 watts mentioned.

I'm driving my 801N with Mac 501 monos and I'm very happy.

"The wattage you need depends on how LOUD you plan on playing them."

I totally disagree with this statement. You don't need high watts to make loud noise. In fact, some of the worst sounding systems are because people are listening to their systems "loud" without the proper power. What you actually want high watts for is for clarity and precision at low listening levels...and of course, then at "louder" listening levels too.