What componant degrades the signal least/most?

The room (did you really expect something else from me?): it necessarily changes the amplitude and phase of a variety of frequencies. The degree to which it changes things and how degrading that change is depends on the room and speaker interaction. A cube shaped room would be one of the worst for amplitude change, as the deviations from a flat response would be huge. A "golden ratio" room (although I don't entirely subscribe to this theory), would give reasonable mode spacing and while it still changes amplitudes of certain frequencies it does so in a more uniform fashion (Bonello criteria). The other part is the phase--reflected sound is always out of phase (well almost always). Only an anechoic chamber can offer no phase shifts--but I don't think you would enjoy music in that environment. Phase shifts are what give us spatial cues.

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.

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