ernestcarl, you are correct....most tests have been with 3D on and have been highly subjective. I would love to see a review with actual measurments.
However, here is my subjective review: I received my Studio, listened to it carefully with the 3D up-mixing on and off, and returned it. While it did some things well, on balance I did not feel it provided an acceptable music experience for me in either mode.
The good: it can play impressively loud with deep bass for its size and price. No question one of them could get me evicted from my apartment without breathing hard. Native 3D audio and Stereo tracks with 3D on do certainly provide a sense of spaciousness and imaging that is much broader than expected from a single speaker.
The bad: In all modes (native 3D, Stereo with 3D effects on, and Stereo with 3D effects off) there is noticeable congestion (blurring, lack of detail, apparent distortion) and tonal unevenness (which could sound honky, distant, or too close depending on mode and track) in the mid-range frequencies. I think there these problems probably exist at all frequencies...but they are most noticeable and less easy to ignor in the vocal region. Whether the stereo tracks sounded better with 3D on or off depended on the track...and they do sound very different. But, in all cases there was a lack of mid-range clarity/detail and tonal imbalance in my room.
If the owners goal is to spend $200 or less on a single smart speaker that can fill a room, and also be used for TV audio...it may be a good choice. In terms of value, you get a lot of sound for the money. However, so far the Studio has not been on sale below the $200 list price. The newest (just released) Echo was recently on sale for $60....you could get two of them plus a refurbished Echo Sub ($80 when available) for the same price. I have not heard the combo...but guess it would sound at least as good.
I think many people will be satisfied with the studio, and it is worth trying. It would be nice if you could turn 3D on or off by voice command (it is buried in the Alexa app with the tone controls...and only visible after an automatic firmware update). Perhaps further firmware updates can improve the speakers accuracy and provide more DSP control options.