@dsper You have put your finger on why lower powered amps also have less gain, since its an expectation they will be used with a more efficient loudspeaker.
A higher powered amp usually needs more gain since its more likely to be used with a low efficiency speaker. When you start mixing the two you run into problems like this.
What you might notice here is that the amps and speakers have differences in gain and sensitivity while the gain of the preamp can usually remain a constant, although in the last 25 years the gain of the preamp hasn't been as important. 26dB is a lot of gain in a preamp line stage these days.