as long as you turn warping of there's no muffled or altered sound in ableton, other than the altering your AD / DA converter does...
the reason your renders sometimes don't sound the same as your project unrendered, has to do with delay compensations in the DAW. ableton struggles with offsetting time on AU and VST units. if you have lots of these plugs on channels, your timing can get screwed in the rendering process. a good way to work around is, is either to render the VST output to audio tracks, or record the master output instead of rendering. many plugs go in oversampling mode when rendered, this causes other delay offsets, ableton sometimes just doesn't recognize them.
also, when you have a very big project, this tend to screw up the delay compensation as well. i notice things becoming slightly out of sync when going above 70% CPU constantly, although i'm not very in the box these days so that almost never happens...
some stuff to think about
PS: when working on a MAC, always use the AU plugs! at least ableton 8 will cause almost always delay compensation problems with VSTs
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