Can a great system make a mediocre recording sound good?

Salesmen always spout what moves product.

Most poor recordings that used to sound quite musical to me no longer do, on any of 7 systems. My ears have evolved. Don’t know about you or your sales person.

Still pretty good on a BT portable however.

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 ...