Can i connect just a slingbox to a set-top box without the use of a TV.
Yes, in fact that's how my Slingbox Pro-HD is hooked up now. It is connected to a Scientific Atlanta 8300HD through the component video cables and the coax SPDIF audio cables. The IR blasters from the Slingbox are pointing to the remote sensor of the cable box to provide remote control capabilities through the Slingplayer software.
It is easier to think of the Slingbox (server) as a virtual television that you watch through the Slingplayer (client).