"rear speakers work only in surround mode"?? What are you listening to? A stereo source? It's quite normal that surround alters frequencies (including those destined to the subwoofer), it's applying processing to audio. To listen to the original source, you have to use standard mode and, of course, you have to listen to a multitrack source to make rear speakers output anything.
If you just want to hear on the rear speakers a copy of the front speakers for stereo sources, I remember there's a way to change how the upmixing is done, but I don't remember how to do it. Samsung introduced this alternative mode in some firmware upgrade. However, after trying that, I still believe that the best experience is to listen to the original source as it was conceived, so I personally always use standard mode. And the subwoofer works fine for me.
