Most of of our current day DAC, preamp, amplifiers have THD far below -70dB, a level which is already very difficult to hear.
( Hint: generate a sine wave at -70dB, and play back on your system at normal listening volume position, most likely it's gonna be hard to detect. )
If THD is already so low for our HiFi electronics, how then can our HiFi electronic equipment affect the harmonic content/structure i.e. timbre?
Of course, speakers, is a different story, as it's THD is usually not that good.