I don't know if this has been covered yet in this thread but, as someone who usually sits off axis, only the Dolby modes seem to keep the centered images in the center. The other upmixers all seen to turn on some form of center spread, which is great if you're sitting in the middle but if you're sitting off to the side, everything just collapses to the side channel as it you weren't using a center channel speaker at all. This effect makes me regret stepping up to a receiver that offers auro 3d, a little bit.
I don't know for certain about newer versions of Auro-3D, but my original version doesn't use a center spread. I think the new ones offer a "movies" and "music" mode like the old Pro Logic II modes and if you don't want the spread, you choose movies even for music.
For DSU models that offer center spread still, there's always an option to turn it off. I have identical L/C/R towers. I leave it off. I cannot hear any difference with it on from the MLP, but I do hear it get worse off-axis, as you say.
Neural X doesn't offer a center spread as far as I know. Older DTS modes had a music mode for that sort of thing. If it does offer it in some models, I'd be surprised if there weren't a way to turn it off or just choose the movie mode in the older versions. I think the only difference in any "music" mode upmixer is the center spread thing.
People like that for systems with poor (or poorly placed) center speakers because putting them on the floor or in a horizontal position (or a special "center speaker" model that must be inferior to the main L/R speakers or people would choose to use 3 centers....) results in worse performance than two high quality regular speakers.
But if you use three high quality regular speakers, it's generally not an issue (there is a difference in room reflections normally that you may prefer one way or another), but using room treatments to absorb the first side wall reflection generally takes care of most of it (unless of course you like that first reflection; I use extracted front wides instead. Sounds come from the soundtrack that are in that direction, not my own walls to the room, which then gives a much wider/bigger image than using the walls).
But if you want the ultimate music upmixer, I still say Lexicon's Logic 7 is the best for music. It's so good, I've got 3 different used units from eBay in my house including a DC-1 in my Carver based ribbon music system, another DC-1 in my exercise room and a MC-1 plugged into my home theater via the 7-input mode on my Marantz Atmos AVR. Expanded with "Scatmos" processing, I actually get a Logic "11" mode (Logic 7 uses front wides and SS#2 here without it even knowing it since Scatmos steered processing occurs after Logic 7 processing). My Auro-3D mode uses Front Wides too because of that. If I had an Auro 13 decoder, it would use SS#2 also (it does in Auro2D mode).
There's lots of (now cheap) Lexicon units on eBay. Any older system with 7.1 inputs can use it as well as HiFi equipment with tape loops and the like (with the Lexicon controlling all surround in that case). You could also technically rig it up with a bunch of RCA mixers if external amps are used for everything.
