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

Good comments by the others. Some additional thoughts:

1)If the impedance curve of the N804 is anything like curves I have seen for some of the other Nautilus models, such as the 801, the 4B-ST will most likely not be able to drive their woofer sections properly in bridged mono mode. Note that the power rating of the 4B-ST in bridged mono mode is only specified for an 8 ohm load, and not for a 4 ohm load. The reason for that is that in bridged mono mode the amp outputs "see" a load impedance equal to the speaker impedance divided by 2. Since the speaker's impedance in the mid-bass region (at least for the N801) is less than 4 ohms, it will be seen as less than 2 ohms, which the amp will probably not be able to handle, at least in an optimal manner.

2)I second the question about what preamp are you using to drive the amplifiers, and can you also describe how the preamp and amps are all connected together (e.g., with y-adapters)? Depending on the preamp design and the interconnection configuration, it's possible that bass is being reduced as a result of either impedance incompatibilities (due to the dual loading) or level mismatches (if perhaps you are using an RCA output for one amp and an XLR output for the other).

3)Assuming that you are biamping passively (without an electronic crossover between the preamp and the power amps), using the 4B-ST in bridged mono mode might result in a gain-matching issue between the low and high frequency amps.

4)Assuming that you are biamping passively, you would probably not be able to utilize a lot of the power capability of bridged mono 4B-ST's, because without an electronic crossover ahead of the amps all of the amps have to output a voltage range corresponding to the full frequency range of the signal. Therefore you would not be able to turn up the volume high enough to use much of the power capability of the higher powered amps without overdriving and clipping the lower powered amp.

Regards,
-- Al