Ineed some help with a simple math equation

The difference in gain will not affect how much power can be supplied to the speakers.  It will just affect the positions of the volume control that you would find yourself using (and a 3 db difference is not a great deal), and the position of the volume control at which the amp reaches its power limit.

Regarding 500 watts vs. 400 watts, that is a difference of only 0.97 db, which corresponds to a volume difference that is barely perceivable.

Regards,
-- Al
 

BUT, saying kilowatt feels so good!

Perceived loudness change can be calculated as:

k^(1/3.5)    where k is a ratio of powers.

(500/400)^(1/3.5) = 1.066  (6.6% increase).
I doubt that you can hear that.  

You want "better".  If you need more sensitivity you will be 2.03dB "better" with 29dB/400W