I always use Dolby surround for stereo content and Neural x for multichannel content.
Stereo is really a flawed format, even for music. The only way to get proper imaging is to sit directly in the sweet spot. With Dolby surround, you get that same imaging no matter where you sit. Dolby surround is a hell of a lot better than Prologic II. I've done some A/B tests comparing a discrete multichannel mix to a stereo down mix using Dolby surround, and Dolby surround almost always manages to extract the surround information that was down mixed with startling accuracy, well enough that I would be hard pressed to discern it wasn't discrete multichannel in the first place. Neural X does a much better job of discrete object tracking with 5.1/7.1 mixes, its nearly as good as real Atmos. DSU tends to add more ambiance, with less overhead panning with multichannel content. I don't use neural x for stereo because it is a comb filtering, channel leaking mess. I have no idea what they did to screw up 2ch upmixing, since multichannel upmixing works fantastically.
The only reason I'd used 7ch stereo is maybe for party music, even then probably not.
Most DSP programs in receivers suck. The only one I kinda like is onkyos theater dimensional. It does a pretty good job of mimicking 5.1 with a 3.1 setup.
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