In the the original question (which removes source material and room acoustics from consideration), the definition of "best sound" was addressed. For some it may not be the case, but from our perspective, "best" means the most accurate reproduction of the source material.
If you look at it from that perspective, the importance of the speaker dwarfs the importance of all other components. The reason is that speakers are the least accurate components in a system. So if the goal is accurate reproduction, speakers are the most critical.
At the risk of oversimplifying, even a moderately priced receiver is rated at 20 Hz - 20 KHz +/- 0.1 db. Speakers, on the other hand, are generally rated at x Hz to x Hz +/- 3db...orders of magnitude less accurate.
You would never consider purchasing an amplifier rated at +/- 3db. Yet we readily accept speaker performance within those tolerances. From that perspective, speakers are clearly the limiting factor where accurate sound reproduction is concerned.
Granted, this is a rather simplistic analysis. But it should serve to illustrate that a rating of 80 - 90% for the speakers is likely most appropriate.
Do higher quality transports, preamps and amps make a difference? Of course they do, but not as much as some would believe. You could drive a $100 Big Box pair of speakers with a $10,000 amp and the results would likely be quite poor. But drive an appropriate $10,000 pair of speakers with a $100 amp and the sound would be surprisingly good. While I am not recommending the latter approach, it should serve to illustrate the point...speakers are by far the most important component in a sound system.
Just a speaker builder's opinion...
- Jim