I think like a engine in a car the amplifier has to be able to drive the loudspeakers and control the motion of the drivers without an sense of stress. So if your speaker has a very benign load then lower power works great. If on the other hand your speaker is a difficult load with low impedance's and difficult phase angles the larger more capable amplifier is the better option.
So a lot depends on the speaker chosen the room size and the listening levels required in a specific setup. I think the reason the 14B the 7B's and especially the 28B's do so well in most systems is because many speaker loads are much more difficult than most realize and an amplifier like the 28B just does not give a crap about what the load is 
james
hey james, i believe in high power (and current) meets high efficiency, especially speakers that dip down to 4 ohms like the jbls i currently use...as far as i know, they should be able to handle the power and current of the 7b, 14b, and 28b cubed. according to techbical support at harman/jbl, they recommended at least the power of the 4bsst, which i also use. i was surprised at this given the high sensitivity rating of 94db...apparently this is what they subscribe to. i think it has to do with the 15" woofer, which has a big voice coil and magnet...
