Ineed some help with a simple math equation

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

As a Thiel 3.6 owner I wholeheartedly agree the more power the better (those speakers are excellent at high level transients, which is probably why they need power).

My suggestion: since the Thiels have an average impedance of less than three ohms from mid bass to 20khz, go with the amplifier that has the highest 4 ohm power rating at 1000 hz. (I assume those power specs are at 8 ohms).