Interesting notion... All of the aspects you mentioned are important, but if I had to order them, here is the order I would pick and my reasoning why:
#1 - The Musicians
Quite frankly, music wouldn't exist without people making it. If there was no music, there would be no necessity for any of the other three aspects as there would not even be a tracking scene. :P
#2 - The Trackers
Although tracking came about as a method of taking advantage of Amiga sound hardware, it's kept its relevance not because of the sound cards, as the sound cards became more advanced fairly quickly, but because of the technical aspect of being able to fit more music into a smaller space without having to resort to MIDI, which even nowdays STILL sounds different depending on the hardware and software being used to play it back.
#3 - The Soundcards
Obviously, you can't play music on a computer without a soundcard, but given that some trackers have the ability to use devices as rudimentary as the PC Speaker, the tech behind the sound cards is not the leading factor behind the quality of the trackers throughout the years, which is why I put this below the trackers.
#4 - The Players
The reason this is at the bottom of the list is because most people who actively download and listen to tracked music likely have an interest in making music and tracking to begin with. If tracked formats were more popular in general and a large number of average people listened to them on a regular basis, media playback programs would support the various tracker formats more readily, but alas, that's not the case. Not to mention, you can play back tracked music in tracker software itself and some players which can render multiple tracked file formats don't render all of them 100% perfectly, the IT format being the most notorious given what was done to it by future IT-compatible trackers.
Again, all four aspects are important, but this would be the order I would put them in.
--- Kris Asick (Gemini)
--- Pixelmusement Website: www.pixelships.com
--- Ancient DOS Games Webshow: www.pixelships.com/adg