Can a great system make a mediocre recording sound good?

Nope. A great system makes a mediocre recording sound mediocre.  There are lots of systems that make everything sound mediocre (and give the illusion that mediocre is somehow good), but that doesn't make them great systems.

If you ask an acoustician he/she will tell you that 80 percent is the room and 20 percent is the system (if not 90/10). Something to think about when you judge a recording being bad. Maybe its you and your system/room that is at fault. I’ve tried to learn the lesson and agree with others that have pointed this out. A good system should make any recording sound interesting. It’s an easy task to make a Steely Dan recording sound sweet.

+1 @gosta 

Well said, my experience exactly.

There is still lots to hear in even a mediocre recording and a better system will do a better job of delivering what’s there.  That means a better listening experience and a better listening experience means the mediocre recording just got better. 

I've found that most 'poor' recordings only sound poor because the system/room I was hearing them in was not good enough to translate the acoustics of the space the recording was made in.

Exactly.....