The gain is in the spec sheet. For the 7500/7700, it's given as XLR 28dB, RCA 34dB.
Now to the strangeness: for power, 10 dB is 10x the power. For voltage, 20 dB is 10x the voltage.

Again from the spec sheet, input level for full power output is 1.34 Volts.


V is voltage. I is current. R is resistance.
P is power.

Power output of the amp is not linear with respect to input power. First, there's some idle power (bias currents, transformer losses, etc.) that is consumed even with no input signal.

There are other factors that depend on the particular class of operation (A, B, AB, C, D, G, H, etc) of the amplifier and are beyond the current scope of this discussion.

In actual practice, 91 dB is pretty loud (typical speaker efficiency at 1 watt) and most listening is below this level. The extra power is to accurately reproduce dynamic peaks in the sound without audible distortion. Back in the days of vinyl there were a few limited edition pressings that had as much as 70 dB dynamic range. Some of these had peaks 35 dB or more above the average music level. Assuming 80 dB average, you'd still need around 300 watts output to handle that peak level.

Also, on a speaker rated to handle 100 watts, the tweeter may be capable of handling 20. Overdriving an amplifier (clipping) produces a lot of distortion, most of it at high frequencies.
A 20 watt amplifier driven into hard clipping may produce 40 watts of power momentarily, most of which will go to the tweeter. This can let the magic smoke out.

(all electronics run on magic smoke. PROOF:
when you let the magic smoke out, the device quits working.)

Steve