I agree with others that bass on your N804's will always be limited. :( Still, a bigger amp or biamping can't hurt.
I drive N803's with an (ancient) SAE s/s amp. 200wpc into 8 ohms rising to 300wpc into 4. The old thing has ample dynamics and virtually never clips. Whatever you pick, make sure its output increases significantly into more resistive loads. All the Nautilus are only nominally 8 ohm speakers. They can drop as low as 3, which puts a huge demand on the amplifier's current delivery.
I drive N803's with an (ancient) SAE s/s amp. 200wpc into 8 ohms rising to 300wpc into 4. The old thing has ample dynamics and virtually never clips. Whatever you pick, make sure its output increases significantly into more resistive loads. All the Nautilus are only nominally 8 ohm speakers. They can drop as low as 3, which puts a huge demand on the amplifier's current delivery.