@redlenses03 "Could you expand a bit on this. More specifically, maybe provide an example for a good match vs. a bad match WRT gain matching a pre and amp (stereo or monos)"
Amplifiers provide gain, either in voltage (typically preamps) or in current (power amps). Amplifiers also all generate some degree of noise, have an input sensitivity, and a maximum output level before clipping.
Signal to noise ratio is typically measured as the noise floor below some reference level, e.g. 1 volt for preamps,1 watt for power amps. Dynamic range is measured from the clipping level to the noise floor. Similar, but not identical.
Since the OP was asking about an MC-275, I’ll use that as the example. The McIntosh MC275 power amplifier has an input sensitivity of 1.2 volts unbalanced and 2.5 volts balanced. The MC275 also has an input impedance of 90,000 ohms unbalanced and 180,000 ohms balanced. This means 1.2 Volts of output from the preamp will drive it to full rated power - 75 Watts. That's roughly 19dB of gain.
OK, here’s where it gets a little tricky. Suppose your preamp has 10dB of headroom above the 1.2V needed, that’s 12 V. The noise floor is (lets say) 60 dB below the1.2V, so that then becomes the maximum dynamic range for the system. Worse, the S/N relative to 1 W output is reduced by the 19 dB to 41dB. If I turn the input sensitivity of the MC-275 down, to where it takes all 12 V from the preamp, the noise floor stays the same on the preamp, but because I have to drive the amp with a higher signal level, my S/N improves by, in this case, 10dB to a more respectable 70 dB.
Now, if you’re driving LaScalas with 100dB 1W sensitivity, it doesn’t take much signal to make an audible noise, and 41 dB below 100dB is 59 dB. easily audible in a typical listening room. So, a poorly gain-matched system can easily be heard as noise, or hum, or both. Reducing the input sensitivity on the amp and driving the preamp harder moves the entire signal further above the noise floor and the system is quieter to the ear.
There’s more to consider, however. If you’re listening to vinyl, there’s another preamp stage - the phono preamp, and it’s gain structure needs to be accounted for. Worse, if you have a low-output moving coil cartridge, whose output can be .5 millivolts or less, you need another pre-preamp stage which carries its own noise levels and dynamic range, and that must be adjusted for the output of the particular cartridge being used.
So you can see how it’s very easy to end up, even with very high end components, an improper gain stage design that results in an audibly noisy system. In recording studios, engineers routinely lose sleep over their signal chains trying to maintain proper gain-staging and a quiet environment. And this is before you consider grounding issues and environmental electrical noise on powerlines, or introduced by inadequate power supply filtering.
There are whole books and courses on this subject, and for good reason. However, I’m going to stop here. Hopefully though you see the basic idea - raise your signal above the noise floor, without clipping the upstream source, at every opportunity. The best you can do with your ears is keep the input levels on your amp down, and drive the preamp harder, but not so hard it clips before the power amp. Doing this accurately however requires some degree of technical savvy, and test equipment, including signal generators, dummy loads, and most importantly an oscilloscope so you can see the noise and clipping levels.
