They are selling all Heybrook stock at 30% off, which may not be a terribly good sign.
The original owners of Heybrook (husband and wife) retired and sold the business mid 90's. The Heybrook name then became part of a group of (I think) 3 other brands, based in Plymouth, Devon, UK. Not sure what has happened to this group but I think they may have folded. The last contact telephone number we have is +44 (0)1752 333800
I remember Heybrook used Tonigen tweeters until they became unavailable. I belive they then used Audax units.
You may be able to get some info on the tweeters from Audax (www.audax.com)