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
 

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

Better measurements IMHO are distortion and noise in the actual usable range. If you have spec sheets, look at the THD+N in the 30-100 W range.

However, inefficient speakers are less sensitive to noise, and many doubt we can hear anything below 0.5%, so absolute numbers below that may e meaningless.

Best,

E

Also important to understand how they eat power. Are they low impedance ? Difficult to drive due to high phase angle or low Z in the bass?

You may indeed need a very solid amp, but not necessarily need all those rated watts at 8 ohms.

Best,

E