Clearly, an untreated room could add the most distortion to the signal. I have seen 10 dB swings in the room response.
Your front speakers may be a bit more important than other speakers. Sure seems like you would want timbre matched left, center and front. Speakers are a bit of an objective device, because there's compromised involved in making them. Clearly all speakers are important, because they are what you hear.
A subwoofer IS a speaker. But it's also often a powered speaker. So you need sufficient power. And ideally, it would not have too much distortion. The higher the crossover used, the more it would seem important to minimize distortion.
Electronics are overrated. Primary goal is usability and sufficient amp power. You can argue all day over stuff that can't be quantified by measurements, or you can just trust that reputable gear has distortion lower than any of the above factors. If you can hear the difference between two preamps, DACs, amps, etc., and have the money to buy the better sounding (perhaps not even the most accurate gear,) then do that
Personally, I avoid low end receivers because they have small transformers, and seem incapable of sufficient power. If you buy a receiver with enough power, you may have little to complain about - but some audiophiles insist they must be bad, because they don't cost a lot, and lack a prestigious name on the front panel.