I’m actually surprised that the input gain of a preamp doesn’t really effect the output signal gain of the amp. I thought it would be additive or multiplicative (if that’s a word). Can someone explain why that is?
Amplifiers are fixed. All an amplifier does is take whatever comes in and multiply it by some fixed amount. That fixed multiplier is the gain.
But it can only do this to a certain point. In your case 30W. At that point it doesn't matter how much more you input, or how high you turn up the volume on the preamp, that amp is not capable of putting out any more power.
An active preamp is really nothing more than a small amplifier with an attenuator (aka volume control) and some knobs to let you switch between sources. That small amplifier is fixed just like the power amplifier is fixed. Same deal.
All your sources connected to the preamp, they all are at line level. Line level is a couple volts. (The exception is phono, which is in millivolts, which is why turntables need a phono stage, to bring millivolts up to line stage voltage levels.)
Now here's the thing- all your line level components have more than enough power to drive your power amplifier to clipping, ie as loud as it will go.
So in practical terms the gain of the preamp is irrelevant. More often than not its not amplifying anything, its attenuating.
If you have something like a CD player with variable output you can prove this yourself by simply connecting it directly to your power amp. It will drive them just as loud just fine. It will probably not sound quite as good. Why? That gets a little more complicated to explain.