I have the exact same speaker setup that you do. You are right about the Nautilus speakers being power hungry. They sound OK at lower power but every time I have increased the power to them it has made a HUGE difference. Greater level of detail, larger soundstage, but most especially in overall dynamics. The more power, the closer they get to real live music in your listening room. Another good news/bad news trait is that they are very revealing of any strengths or weaknesses in the rest of your gear. Of course that's what keeps us on this endless upgrade path, right?
I am using bi-amping for all five speakers. The bass portion of each speaker is powered by the speaker outputs of a Denon AVR-5803 receiver (170W/channel @ 8 ohms), and the mid/tweeter portion of each speaker is powered by a B&K 200.7 amp (200W/channel @ 8 ohms). The pre-outs of the Denon receiver for all five speakers are run to the inputs of the B&K amp. It may seem like the bass portion of the speakers should get the larger amount of the power, but I tried it that way and it sounds much better with the mid/tweets getting the lion's share. Also, I use a Velodyne sub for everything below 60 hZ, so that the bass portion of the speakers don't have to work as hard as if they were being asked to go to the low, low frequencies. Bi-amping seems to give a much cleaner sound than bi-wiring. I have tried it both ways.
I talked to B&W about this setup and they encouraged it. I was concerned about overpowering any portion of the speakers, especially the 804's or the HTM-1, which are rated at a much lower power than the 802's, but they said it would not even come close to overpowering the speakers. One of their engineers said that the biggest danger to speakers such as these is to underpower them. If you are not going to use a receiver, and only a pre-pro with an amp, you could possibly use a pre-pro with two sets of pre-outs. I don't know if there is such a setup available. If there is, then you need to make sure that both sets of pre-outs on the pre-pro are output at the same time.
I am extremely happy with the B&K amp, it is extremely clean, detailed, and dynamic. And made in the USA.
Also, I run everything through a Monster HTS5000 Power Conditioner/Filter that turns on when the receiver is turned on and then turns on the B&K amp and the sub with a slight time delay. The thing that made as much difference as any gear upgrade or boost in power is adding a dedicated 20-amp breaker using 10 AWG Romex, strictly for all of the home theater gear. Going up to that much power with the amps, etc. requires a larger source of clean juice.
BTW, I also do a lot of two channel listening, and once you get the 802's and your new amps, IC's, etc. broken in, you will be amazed. I use CD, SACD, DAD's, LP's, and occasionally DVD-Audio as two channel sources, and the sound quality through the 802's is truly awesome. Not just my humble opinion, but confirmed by many other listeners, including musicians. Even the wife now sits down and enjoys the music.
Good luck with your search and have fun with it.