Gain / Amplification

It’s about matching the volume of various inputs so you do not need to adjust the master volume much, certainly avoid any loud jump from an input change

Often you are talking about more or less attenuation rather than gain.

Much equipment, like my McIntosh Preamps SS C28 and Tube mx110z have ’trim’ controls for the phono and aux inputs to match them to each other. They were located near the front on the top, you pull the unit forward a few inches, adjust them and push the unit all the way back.

My Tandberg Receiver had trim controls in the back.

Any unit I have had, like Onkyo DX-7500 CD Player

https://www.hifi-classic.net/review/onkyo-dx-7500-430.html

had 3 outputs, rca; direct; and variable. I couldn’t hear any difference, neither could my friends.

Likewise, I had 3 oppos, and the output variations sounded the same to me. Turning off the display may be measureable on the bench i.e. .00002, but its marketing IMO.

Gain usually comes with additional noise. The less gain you have overall the less noise.

Phono stages have the most gain, and most prone to noise, followed by line level preamplifier circuits.

That is all in theory though. Over the last 20-40 years solid state has gotten very very quiet so it’s not usually worth sweating these details vs. say room treatments, listener location, etc.

The one pet peeve I have though is that most preamps still have WAY too much gain, maybe due to the days when radio might pick up a weak station and you had to compensate for it with the volume control.  If I used a tube pre especially I'd be tempted to reduce the gain quite a bit.

is there likely a quality-of-sound difference between utilizing maximum gain (say, out of a CD player, to keep it a simple scenario) and needing much less amplification from your amp, or is it in any way different to set your CD gain/output volume at a lower level and let your amp do its job?   Let’s say balanced connections, so that’s added gain as well.    

If the balanced connection supports AES48, the balanced line standard, there may be no difference of signal level between balanced and single-ended.

The newer more sophisticated digital volume control systems are a bit better at resolution. Older ones cause a loss of resolution as you turn them down since bits are subtracted.

This is why there is a market of line stage preamps since they often have better volume controls as well as often are able to minimize cable interactions better than most sources.

Years ago I did exactly what you are asking. I plugged my Oppo straight into my amps and it worked just fine, but because CD’s were not the only thing I listened to, I went back to my ARC tube preamp, that not only had input switching , but had a different sound that I preferred.

All the best.

This is interesting.  I have a server with software interface volume setting for fixed or variable feeding my DAC ( no vc). Although variable control would be more convenient as my tube preamp has a manual stepped attenuator I set software to fixed output. I've listened both ways and even eliminated the preamp from the chain without noticing much difference. I've thought digital software driven volume control was supposed to be inferior? I like having the tube pre in the mix although unnecessary in my system so it's acting more like a buffer with gain. By the way my preamp has 12db gain. I tend not to go back and forth between configurations so I'm used to getting up to manually adjust volume.