No first hand experience and I was compelled to look up that model.
What I read was those HS-400's were part of Hitachi's Lo-D series - an upmarket line from their regular consumer stuff. Was also not widely available in North America.
This is an excerpt from a site called Audio Database:
The 2 way speaker system which adopted the metal diaphragm.
The 20cm metal cone Woofer which sandwiched the foaming resin is carried in the middle of a cone low-pass.
Moreover, the sailor cap type 3.5cm titanium tweeter is carried in the high region.
The gathered edge is adopted as each unit.
Direct translation from Japanese so I've no idea what that last sentence means.
I'd almost think Hitachi borrowed the idea for that woofer cone from Leak's Sandwich series
For a Japanese speaker, they should be relatively good sounding. And scarce too which appeals to me. Not suggesting they're worth big bucks just that I have an affinity for the oddballs.