If we want the music system to offer high fidelity, then we mean it should reproduce the signal the way the recorded media sends. Notice that RECORDING the signal (in ther words encoding the live event) is also done by an engineer, who may equalize certain aspects of the performance to produce effects. Nevertheless, the goal of the sound system is to REPRODUCE this signal, not to reproduce the LIVE event.

A system at home that can reproduce the signal as it was recorded MUSt reproduce all frequencies flat, and offer neglibile distortion. Otherwise it is adding its OWN signature.

Now comes the subjectivity: some systems are not perfect (surprise!) and do add their own signature. In some cases that signature will enhance the recording (for example the second order distortion and non flat freq curve of a SET amp may make a female singer's voice a little more life like) so it appears more lifeliek to us, even more lifelike than perhaps the recording engineer intended. By the same token, the same amp may distort another recording to the point where it sounds pretty bad. (try playing ZZ top with a SET and single driver set up!)

Overall, if we are concerned about fidelity, then we need to be concerned about simple measurements. If we are concened about just how recrodings sound, then there are a lot of variables involved (liek what is the recording, what effects were used ,etc) and systems with poor fidelity will often enhance some recrodings and worsen others.

It's really all a matter of taste, finally. I am at present trying to pursue fidelity, but in my midrange (dfrom 60 hz to about 3500 HZ) i do have a set powering a high impedance driver! However, i am using a cheap sub for the below 60 hz a,d a cheap tweeter fr the treble, all powered by SS! The overall reposnse is far flatter than just using a SET with a full range driver, and to my ears more pleasing (much more pleasing actually). In other words I can now listen to both PAtricia BArber AND ZZ top on the same system ,and actually really enjoy both!
Not to mention complex orchestral arrangements!
-akhilesh