The trouble with this "it's not the system it's the room" argument is that the good recordings and the mediocre recordings are both being played in the same room. Moreover, it is the same room as my upgraded-from system. I fail to see what difference the room would make in comparing one recording to another in the same room. Judging from what some people spend on gear I'm sure some of them could afford different rooms to listen to different records, but that is not a lead I am in. A better system makes all recordings sound better than they were, but they all finish in the same order as before in terms of recording quality.
The Ry Cooder record is indeed a good choice, but I made it back in the 1970s when I bought it. Speaking of good choices, one thing I learned now that I have heard just about all of the original versions of the songs he covered, that whatever you think of him as a musician, he sure knew how to pick them. Something very interesting could be written about folk singers as music critics, based on their choice of material.
The ultimate example of the difference digital mastering can make is a comparison of the first, second and third generations of the Complete Robert Johnson, the last of which is an absolute revelation. I think they might have had the original metal parts for that one. The ultimate test would be if someone had a pristine set of original 78s to compare it with, possibly to be found next to the Arc of the Covenant in that big warehouse at the end of the Indiana Jones movie.