You will not get significantly more headroom. If you double amp power you only gain 3 db. With passive bi-amping you will not gain anything like that much. This is because, the low pass section takes most of the power, and the high pass comparatively little. The power divide of music is in the neighborhood of 400 Hz. However that is where the baffle step compensation sets in, which requires a 6 db increase below that point. So to all things being equal, which they may or may not be, you need three times the amp power below 400 Hz, compared to above 400 Hz. So if you already have a powerful amp, you have to massively increase the available power. To get 10 db of headroom to keep things really relaxed then you need 10 times the power. So that is why in my main theater I use 3,200 watts all channels driven. That gives me10 db of headroom over a 320 watt system.
This is an area where things are much more complex than you think. Fortunately with reasonably sensitive speakers 40 or 50 watts per channel gives most a satisfying home experience. However if you want the full concert hall experience, with the full dynamic range, and have things stay totally smooth and effortless, then you really have your work cut out and then some. To put it bluntly, it requires a significant degree of insanity.
