Gentlemen:
Read this again:
"That's what I use to judge how large an amplifier is required."
There is nothing in this statement that suggests 400 watts are required for you and nothing that suggests quality is based upon 400 watts. Those inferences drawn above are incorrect. I use this method and it is common in the studio field. What you do is your business.
I suggest, instead, you read Porscheracers link above. It provides specific details about power needs.