Which component is most responsible?

It's how it was recorded. Either the recording has or it doesn't. If it does, even a moderate quality system (with attention paid to setup) will reveal this effect.

Onhwy61 beat me to it, but to hear distinct separation of vocals requires the microphones to be placed in the optimum locations to achieve that effect.

It always tickles me when someone defines this or that component as having great imaging because they can hear the location of each instrument on the stage. If it was not recorded that way, you will not hear it that way.

Sticking my neck out here, but with all things not being equal, but some things being more equal than others, it is in most cases, the spealers. Why? Refinments in electronics has been ongoing for at least a couple of decades. While some may argue there is still a difference between an amplifier that has .1% distortion & one that has .05% is significant, I've yet to find one person who can distinguish & identify a particular amp based on normal conditions, based on distorion alone. Average output level, say 3 watts or less. Playing it to extremes some differences might present themselves, but there should not be any noticable difference based on distortion alone. Speakers on the other hand never have had, in spite of what one might like to think, such a level of performance since their inception.

Example: If you believe some speakers are unlistenable, how can it be that we can and have listened to a lowly transistor radio for hours or days at a time and still managed to recognize a familiar voice or instrument? And we did it without any disruption in/to our sensibilities or outrage for what we had to "endure."

That cheap tape recorder of days past will still allow us to recognize with out any doubt our own voice, or that of family members & a host of other things.

Take a given source, & try a couple of dozen different speakers, and you're going to hear the subtlties of that same dozen speakers. Ultimately you'll find some that are more to your likeing than others. No two people or two speakers are the same. You may succumb to the suggestions of others. Some may make valid points, some you might confirm or not.

Now, make you best choice, put your money down and get on with life. Once & a while though, our insecurities overwehlm us.

I'll be in the minority here, but my ranking would be.

1)Speakers
2)Recording Quality (If it's not on the material, it's not there.)
3)Source (turntable quality is obvious), with digital there are differences in the quality of the DA Converters and how they sound on the analog side. This is why people spend gobs of money on expensive DACS.

4)Electronics (I'm probably in the minority here, but I don't feel amplification should add any coloration to the source)