i honestly don't see the purpose in bi-amping, most home setup amplifiers have more then enough power to make your ears bleed.
and as far as the amps making a difference in sound, its possible, but not likely, most amplifiers sound the same unless you have a bad amp, if one of your amps has better sound then the other, it probably means that the other one is a piece of crap. i have two receivers, a kenwood and a marantz, the marantz sound 10x better, but i also know the kenwood is not a good amp because it for one blew out my tweeters from clipping. oh and as far as only supplying 15w to the mids and tweets, most of your power is used for low freqeuncys, the mids and tweets usually use about 20% of the power and the woofer uses the rest for bass output, low frequencies take more power because to create them you have to move alot more air and the movements have to be much wider to generate a longer waveform, you will notice you cant visibly see you mids or tweets vibrating, but you can easily see a woofer "pumping" in and out.