If you're having sound quality problems with Ableton it's most definitely the options you chose, and also if the song was warped (any of the warp modes besides repitch will absolutely change the sound of the track, which is why you have to be careful about what you warp when you're producing). This article shows pretty clearly that Live's summing bus itself has nothing wrong with it: http://www.minzmusic.net/blog/…/ableton-live-summing-bus
PS I use Live, I used to mess around in FL Studio and now I'm thinking about getting it again just because it has so many sound-destruction tools and some cool stuff Live doesn't have (you can route midi-triggered envelopes to pretty much anything, as far as I know). I wouldn't use it as my full DAW, just as a VST in Live probably.