BIG speakers!! :^) Seriously, while any system can play Mahler symphonies and be satisfying in its own way, in my view to get the full emotional impact of Mahler you need full range speakers (nearly flat extension at least into the mid-30s (bass drum fundamental is usually around 40 hz) and which can move a lot of air as well at those frequencies) with big power handling capabilities, and electronics in front of them that can deliver the goods for the huge dynamic swings Mahler's music contains. My speaker system works very well with Mahler symphonies, but it's not really commercially available yet so you can't audition it (unless you come to my house!) and it isn't everyone's cup of tea. Think big Thiels, Dunlavys, Dynaudios, Vandersteens, the big Maggies and Soundlabs perhaps--speakers that can create a large soundstage, give deep bass and move a lot of air. And you'd need a big room to go with them as well! Or try a good pair of headphones??
