Hmm. I just tried a comparison where I faded all the way to the rear, then flipped between Dolby Surround on and off. With Dolby on, the rear speakers produce a much weaker sound. With Dolby off, the apparent volume and frequency range from the rear speakers is increased.
However, when the sound is faded to my preferred position (-4), it actually sounds somewhat better with Dolby Surround on. I think this is as intended: Dolby Surround takes the normal two (stereo) audio channels and processes frequencies to simulate a 360 degree sound field (i.e. some sound comes from behind you instead of in front). The two rear speakers become the satellite "surround" channels, instead of just reproducing the front left and right channels equally in the rear as you fade back.