I have a number of the recommended recordings, but I don't understand how studio recordings could demonstrate soundstage. Studio recordings have no real soundstage. If there is any, it's only an artificially simulated one. Tried the DiBangogreat music, nothing special with the soundstage. Tried the Micky Hartbetter for soundstage, good depth, but still nothing special. Put on Sting's Soul Cages. The soundstage is amazingly wide and consistently several feet beyond the speakers. Sometimes it even sounds like 5.1. The engineering is spectacular. What's going on?
Well, in the liner notes there is a reference to QSound, a virtual surround process that explains the amazing soundstage effects.
Go to: http://www.qsound.com
Other artists' recordings using QSound: http://www.qsound.com/spotlight/users/recording-artists.htm
Still, the best recommendation for demonstrating a real soundstage, rather than simulated, would be simply engineered orchestral music. Reference Recordings are a excellent label. There are others, but it's hit & miss.