The IMF website answers some of your questions. http://www.imf-electronics.com/Home/tdl/studio-4
They are part of the Studio range, not the reference range. They do pre-date the Richer Sounds stuff by quite a way.
Value-wise I'd hazard a guess at around the £500 mark, give or take a ton. I'd expect them to outperform new speakers up to at least 2 grand, but big, old floorstanders don't tend to sell for much, with the odd exception (Ditton 66 for example). A lot of speaker for the money for someone who has the space to make them work.
Current Lash Up:
TEAC VRDS 701T > Sony TAE1000ESD > Krell KSA50S > JM Labs Focal Electra 926.