Guido,
Can you quantify in what way the newer model would outperform the older ones?
Of the 3, Ref1000m would still seem to have to set the mark for wattage.
600 is newer Ncore versus the others older Icepower, so apples/oranges there.
I would expect newer Class D modules within a line should outperform older in terms of bandwidth, etc. as the technology improves.
Then there is always how each "sounds" which is more subjective versus performance which typically must be measurable.
I own ref1000m. How it sounds exactly is a function of teh entire system playing. How it performs is not.