Anybody says he can place a stable image behind you with front speakers only( if he is not using your personal HRTF in the signal processing) is lying

Front/back distinction is dependent mainly on individual HFTF differences, and HRTF generalization is not really successfull.
The OSD is NOT a virtual surround from 2 speakers, although Marantz had to include that for marketing reasons. It is a very well thought out tech to solve the transaural problem. The others are either based on the false assumtion that they can extract and enhance the ambience info from a standard stereo record, or because the geometry constraints very much depend on individual hrtf, and suffer the non invertable transfer function problem. The better of of these thechnologies I've heard is the Stereo dipole (ISVR also) and the Dolby Virtual Speaker in stereo only wide mode. But they have all the problems OSD tries to solve.
The OSD requires minimal signal processing and complyes with all the other requirements - like high lateral differences in the bass range, directionally correct treble for center images etc. - for lifelike sound reproduction.
Concerning the sweet spot I do not really care as hi-fi reproduction ( the beeing there sound) is hard enough for one - max two - person. If I want good sound all over the room, that is very different requirement, but it can never reproduce the accuracy and intimacy of the former.
If you want real surrond you have to duplicate the mid-high section at the back and feed with the processed rear channels.