Have yet to hear DSP work well for 2ch. It usually make the soundstage smaller, and makes everything sound compressed. I'm sure it's smoother, cleaner, optimized, but it takes all the emotion out of it, feeling sterile.
Yes, DSP for my 9.4.2, (13 speakers @ 150wpc) absolutely! It's almost impossible to time align all them speakers to a listening spot. Soundstage isn't affected with 4 speakers providing it. But music (unless encoded for atmos) sounds lifeless, and dull, but movies are incredible.

