I have 2 pairs of sennheisers and I don't think they can be beat. It's amazing the difference in recording/producing from modern stuff (overdriven crap) to classical stuff where recording quality really truly matters. So be aware, the higher quality of stuff you get, the more pure the sound will get. In many cases of modern music, it will end up sounding worse. http://en.wikipedia.org/wiki/Loudness_war
Got to reiterate this, even entry-level audiophile systems will make lots of modern recordings sound like a dog's ass at anything above a whisper. On the other hand older pressings and music that sound very dull on cheaper systems can be absolutely amazing.
Does that mean new stuff sounds good on garbage and old stuff sounds good on good equipment?
New stuff doesn't really sound good on anything. By new stuff, I mean poorly produced stuff, which almost all music now is. Good equipment is more faithful to the original recording- for better or worse. They don't have the "super duper bass" or anything. So if it was recorded/produced poorly, that comes through even moreso. If it was done really well, it will sound even more amazing and clear.
Here's an example of poorly produced music-
Death Magnetic is notoriously loud. The Guitar Hero tracks are pre production. Note how clear those are.
Good systems are very revelatory of what's actually present in the recording but as ew said: that faithful reproduction may be a good thing or a bad thing.
Good systems are a definitely double-edged sword, there are albums I love (e.g., much of The Doors and old Sabbath) that have very noticeable flaws that are annoying and really stick out now but which I'd never really noticed on cheap equipment. On the other hand great recordings and great masters really stand out; check out e.g., Steve Hoffman's master of The Who's "Who's Next". Simply amazing.
For an entire genre the most striking difference, for me at least, was late 60s & 70s classic rock much of which I thought was a bit dull-sounding. Listening to some of that on good equipment for the first time some of those tracks have just amazing depth and detail that I'd never noticed before.