I have sympathy for those who own the D series telling us they are not off putting or bright.
After all: I own Thiel speakers ;-)
But I can guarantee that no one listening to the Thiel 3.7 or 2.7s in my system would be putt off by "brightness." They've converted more than one Thiel skeptic.
So I can see the Spendor D series being tamed, or working in some systems.
Still, in both encounters with the D7 they had that same hard, steely, off-putting character in the treble, and when I heard the A series they also had a similar off-putting character in the highs. But, horses for courses...