Can a great system make a mediocre recording sound good?

The best you can hope for is for the system and any given recording to sound like it did when it took place...good, bad, or indifferent.  

@gregm 

If by mediocrity you mean bad mastering, I beg to differ.

If it is really bad mastering then sure, I agree with you. But how do you know unless its heard and verified by a great system? But even then, I suspect it will still sound better than with a lesser system

Tune your system so it plays 90% of your recordings well. That is the key. If you have a highly critical system, you will only be able to listen to 1% of your collection. Tune your system to satisfy yourself with the music you play. 

considering that the best music (from the one recorded on media) was already sung in the 20th century and hardly anyone can surpass it (now we have a monstrous degradation of performing skills) - I would not look for systems with a very high resolution ... CD is a good format.
With age, you will still come to headphones and YouTube - the content of the song will become the most important factor for you ...

I've owned a CD for years that I could never find listenable on any of the systems I've owned in the nearly twenty years  I've owned the disc,  a Vanguard Classic  recording of J.S. Bach's Brandenberg Concertos,  by the English Chamber Orchestra, Johannes Somary conducing.

I was able to listen to both CD's in the set, and actually enjoy it.

I'm inclined to say the system can have quite an affect on how music sounds on it.

Regards,

Dan