How much power is enough for B&W Nautilus 804's?

You've got loads of power in the 804s...loads. The 4B-ST is a mighty powerful amp, reknowned for its bass power. You're already bi-amped with another Bryston.

I would suggest if you wish for more powerful bass, other than room placement, you may wish to go for bigger speakers or a well-integrated sub option. If it is simply bass you seek, there are some very good options that will take time to integrate...but can be well worth it.

Aside from B&Ws own subs, you could try JL Audio, Velodyne DD-18, possibly REL.

Well, you have certainly proved power is not the problem. IME you have the wrong speaker to achieve the results you are looking for.

I love the 804. i have the 804"s" models... i use mine for mostly home theater so the low of the lows are from a sub... the 804 has really good highs and mids.... buy a good sub... Save the money on the new left and right speakers and just get the sub.

what are you running as the preamp? making the settings change might help as well...

my buddy is a bassist.... if you liked the sound from a commercial massive speaker then it will be hard to reproduce that same sound with anything but those types of speakers.

when it comes to bass size will play an important role... it is not the be all end all though.... but if you are used to a 15" black widow type speaker it will be hard to match it with something from ANY floor standing speaker.

Bill

No matter what you do to 804 they won't have bass even with kilowatt per side. Won't go anylower than 48Hz.
Running amps of different power puts them definitely out of tonal balance.
If you want to get bass the only way with 804's is by adding subwoofer