My findings, and as a result my suggestion when the topic is raised, is that all of the 2-channel material shows at least some benefit from surround processing such as DPLII.
Although I don't listen to that type of music, it's been mentioned that many pop recordings have little or no "natural" out-of-phase ambience and wouldn't show a benefit. Through pop CDs borrowed from the library, online samples, etc., I found that this wasn't the case with any stereo material, which all had enough such content(for whatever reason)to give worthwhile results. My suggestion has therefore always been to take advantage of the available equipment and not voluntarily limit yourself to listening only through front speakers.
