What componant degrades the signal least/most?

Mechanical/Electrical transducers are the problem.

1. The microphones used to make the recording.
2. If you play an LP, the pickup.
3. The speakers.

IMHO These account for about 95% of the sound quality, assuming that the electronics are at least midfi stuff (Denon, Rotel, Adcom, NAD..etc.) Getting that last 5% costs all the money, and is a complete waste of time if the original recording wasn't top drawer.

PS: Really good or really bad room accoustics can play a big part even if the overall quality isn't great. I have mentioned before how good dixieland jazz sounds when played through honky horn speakers in an unfinished cellar.
Some professional performance halls have accoustic panels that can be raised and lowered so as to alter the hall accoustics to suit the particular performance format.

I'm just bringing up a point, really. Circadian rhythms and noise external to the stereo, make a night and day difference with my signal loss. More so than with any component change, but obviously these factors are my "wall", and not necessarily anyone else's.

Transducers is probably as good as answer there is.

The 120 volt wall current that the amp must shape into a signal strong enough to drive the loudspeakers.

I don't know about signal loss. But how about signal perversion?

If that's the real question, then the answer is definitely the amplifier.

The biggest wive's tale around these parts is that an amplifier simply amplifies the signal.

-IMO

Of all components, the speaker is the biggest problem, hands down. I'm not including the room as a component, but if I did, it would be at the top.