Matching an amplifier and speaker is more of an art than a science. If you go on forums and read on this subject you will find that most people end up contradicting each other. There is no set rule that will work for every speaker. There are lots of factors that will affect this, including the type of speaker driver, the power handling capabilities of the driver, the impedance of the speaker, the impedance of the amplifier, the efficiency of the amplifier, the placement of the speakers in the room, the listening position, the volume level, the source material, the taste of the listener, etc.