I have just completed a new document that explains how to connect the TS-590S to the SDR-IQ receiver.
Go to: http://homepage.ntlworld.com/wadei/ts-590s.htm then click on the link: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~"HOWTO connect the Kenwood TS-590S to the SDR-IQ Software Defined Receiver"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ towards the bottom of the page.This is the first release of the document. Please let me know if anything isn't clear, or if there are any errors.
-- 73 Ian, G3NRW The TS-590S Resources Page: http://homepage.ntlworld.com/wadei/ts-590s.htm