When a matrix decoder like PLII is used to derive 5.1 from a normal stereo signal (that has not been Dolby Surround encoded) whether or not there is alot of sound from the surrounds is dependent on the musical signal itself.
What the decoder does is take the out of phase information from the left and right channels and steer it to the surrounds. For the center, it takes the in phase information, sums to mono, and sends it to the center. So if the stereo signal is fairly well correlated between channels; ie they are nearly identical, there will be little to steer to the surrounds and you get the effect you notice. Sure you can bump up the surround levels if you like but you'll have to do it differently for different music. There will be a difference between different decoders as well as they all use a slightly different algorithm.
If an analog stereo or DD 2.0 signal has been Dolby Surround encoded, then the engineer has matrixed in surround info into the left and right channels and the decoder will pick out that exact info and send it to the surrounds, so there will be more sound going to the surrounds. But still the amount will vary based on how much was encoded in the first place.
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