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.