So to sorta wrap all this up, the gain control is the first thing seen by the signal at the MC2105 amplifier input so turning the gain all the way down is (almost) the same as shorting the input. So, any hiss you hear when the gain is all the way down is the hiss coming from or generated within the MC2105 amplifier. The hiss that gets louder as you turn the 2105 gain control up is not coming from the 2105, it is in the signal going into the amplifier and is being amplified by the 2105 amplifier as it should. I.e., that hiss is coming from the preamp and/or the original source.
With the gain all the way up, the MC2105 has more gain (more amplification) than the MC240 so that the amplified noise going into the amp will be amplified more by the MC2105 (as it should) and so the hiss sounds louder at the speaker. The music will also be louder. If you use the gain control to lower the MC2105 gain to match the MC240 (per post 13 above), the hiss level from the MC2105 should be about the same as the MC240 - same 90 dB ratio between the music (signal) and the noise.
So if you hear a lot of hiss when the input gains are all the way down, then the hiss is coming from inside your 2105 and you may have a problem. (But I've never heard a broken power amp hiss, they usually distort or hum when broken.) If it is quiet when all the way down, the amp is working correctly and it does not "have" anymore hiss than the MC240.