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

More power does not hurt. I was a B&W dealer in the days of the original 801 and found they liked to be pushed. In those days B&W demoed them at shows with an ML 3 which had several hundred watts [and sounded like c**p ] and a Crown [SL 2, I think] which had a nominal 200 watt output but which could pulse 1800 watts at 2 ohms. I had one of the Crowns myself and they worked well. My experience in recent years has been limited to a pair of older 805s which I still have; but from what I have read they still like a lot of power. I would think something like the Musical Fidelity Kw 500 integrated, which has 500 watts at 8 ohms and more at lower impedance's would be ideal. The current 550 has even more power , I think. It would drive both pairs of 801s. I have the NuVista amp of theirs and am very happy with it.

I have a mac with 100 watts and it is true to its rating. To realize any noticeable gains in volume, you will need to massively increase power. Going to 200 watts will only gain you 3db,going to 400 watts will get you 6db. Yes, you can get 500 watt mac monoblocks at a massive price.

With that said, get a really good sub. Even by setting the crossover at 40hz, it will dramatically increase the power available to the speakers. Much cheaper fix and way more effective.

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.