This is only a preliminary look.
My TD6M is finally in it’s box.
My first measurement did show the 700Hz dip. I was not upset. I’m old, and I’ve learnt a few things.
I cut a piece of Sorbothane. This piece had a round opening ‘smaller’ than the magnet assembly. The stretchy nature of Sorbothane allowed me to slide it over the magnet. The Sorbothane now slightly covered the ‘top’ of the magnets.
My thought; this will stop the first reflection from the ‘top’ of the magnet.
Success. The 700Hz dip is gone. Unfortunately there is now a slight rise.
If you give me a few weeks, I will figure it out.
First listen, better than my Accuton C158’s.