Be interesting to have a comparison between PA2X and PA3X, and T4 and T5, but with ALL the mastering compression and EQ turned off.
Personally, I believe a lot of what many people hear as an initial change in overall sound boils down to different Mastering settings...
The best way to see if there really IS any real difference would be to play the same style through each. After all, probably MOST of what we end up playing on a new arranger is still the styles we have got used to, or SMF's we have tweaked to be their best on the old arranger. If there really IS an overall change in sound quality, should be apparent without the mastering EQ and compression.
Then listen to the two arrangers playing the same style with the default Mastering settings for each... NOW is there a perceived difference? If so, you can probably get most of what appears to be the new arranger's improvement by simply using the new Mastering settings on the old instrument.
Sure, this doesn't account for new sounds used in new styles, but, as I said, quite likely most of us don't redo our songbook 100% to always use the best of what's new...
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An arranger is just a tool. What matters is what you build with it..!