I have an aging Onkyo TX-DS575 5.1 receiver. No Dolby ProLogic II, or anything, but it has some surround modes for music. I only use two of them:

5-Channel Stereo mode is great for parties because it uses all 5 speakers and distributes the sound better at lower volumes.

Studio Mix mode adds depth to the soundstage by making light use of the surround channels and adding abiance to the material.

I rarely use either mode, though. When I'm listening to a CD, it's almost always is regular stereo mode. However, I always have by sub (SVS PB1-ISD) enabled, since my mains are M22s and I'd be missing on on the lower octaves of music without it.

When I listen to SACDs, I listen in their native format. If given the choice of 2-channel or surround mix, I'll usually pick surround. I find that a wellp-mastered surround mix really adds to the enjoyment of the material.