If you ever listened to the 'surround' channel of a stereo signal (mono will cancel) and simply phase shift that 'hidden' info by using a few frequency bands and play around with adding and subtracting L and R signals you can add that hidden info.
This usually consists of echoes and ambient signals such as the audience in live performances which are added in the stereo signal.
With this I mean by connecting a third speaker between the two + terminals of your speaker amp.
The surround channel of your average surround system is processed extra considerably.

The reveel has less 'bands' the expensive one has a whole lot of frequency bands.

Very likely it sounds better (to some) with music than extreme test signals.
There is a good chance live performances may benefit most.

Willing to bet that to purists and those that are never satisfied with any system it will sound crap.
Also willing to bet it might sound 'better' to other people as well.

It may be a matter of trying it for yourself... I will pass on it though.
The picture from Marv is very applicable in my POV.

This is how it works          :

Just out of curiosity... has anyone heard the '4k$ bigger brother' (BSG Technologies QOL "Signal Completion Stage") and how did they find it ?

Is it 'better' than the cheap little brother 'reveel' (which doesn't seem to reveal but F.U. instead ?)

I have no problems believing you do not like what it 'adds/does'.

Can you upload a music excerpt (without copyright issues) similar to the test signal ?

Oh my, those things are beat by a $0 trivial open-source ITD panner+crossfeed+mid-side hacked together in a few weekends by myself, except the surround channel, where K + my spatializer is the winner. It's even here on the forum with all the trivial math it uses. The critical part is the ITD panner - that works in low frequencies for positioning. Crossfeed works in high frequencies more, while mid-side emulates speaker toe-in and somewhat tighten the angle. (recommended at 4.5 dB)

Stereo-K alone sounds like wrong crossfeed with reverb. Or a recording of an occlusion disc. Perhaps it could be tuned for correct time delay, since it's far too long and the "cross" signal is audible clearly as a separate one. Center and surround sound very natural there though.
Reveel sounds like a strong mid-side processor, 12 dB? Plus a phase flip.