Lots of questions and too many answers !
In regards to Computer Soundcards that are listed as "PRO AUDIO" you will not find a "BIG" difference in sound quality. Some will even argue you'll find any difference between the stock sound cards found in pc's and PRO CARDS. That being said what you will find is inputs and output configurations that may suit your needs better than another.
I think players will all produce the same quality of sound but again the other things to consider are GRAPHIC quality , LOAD TIMES, KEY CHANGING, and if they can handle multiple types CDG / VCD / DVD / NEO / ETC ETC .
Amps - If all other things are equal - what you will hear with a better more powerful amp is a cleaner sound , but this might be due to more power than a more expensive amp. Again I think if all things are equal ... you won't be able to hear a BIG DIFFERANCE.
Now if you take all the things you mentioned SOUND CARD / PLAYERS / AMPS (LEFT OUT SPEAKERS) and take the cheapest versus the BETTER quality and compare them side by side YOU WILL HEAR A DIFFERENCE BETWEEN the bottom of the line and the top of the line... ( Thats my .02)