Hello Phil,
I'm using windows but this distortion behavior you describe seems very familiar to me, especially when playing chords. I don't think this is related with using a Mac or not. Maybe some changes were made to Decent Sampler between the version you used on your old system and the one you are using now. It would be interesting to know which exact version of Decent Sampler you were using then on Windows and which one you are using now.
As a quick workaround, you could try to lower the Volume of the instrument to something like -12dB or even -18dB in Decent Sampler UI (the volume control in the top black band)
While being lower the instrument should not distort anymore (I think)
I don't know the inner workings of the software but from what I could observe when assembling my own instruments, there seems to be some dynamic compression or non-linear effect at work in the engine when the sound is too loud.
And it sounds (and looks in a sound editor) like distortion as opposed to simple hard-clipping.
I wish this could be disabled at the instrument level. If, as I suppose, Decent Sampler outputs audio to the host application (DAW) as floating point data, the clipping does not really exist and can be easily recovered just by lowering the volume. Any other VST instrument I used works this way.