Now, this is topic dear to my heart, so thank you ;-). We are aligned on native formats comparison.
Re upmixing - I think of it as “active acoustics”, complementing Planar Bass/Waveforming. My target is not to get NeuralX type of algo, but to extract exactly this envelopment and ambience out of stereo. Technically, I am using individualized Auromatic based algo, as it has best L-C-R decoding, with 17 channels [incl LFE]. It was first setup by calibrator and meanwhile I have learned which lever to push to get what I want. Trinnov allows for flexible adjusting - e.g. I can address my Front WIdes, via Auro, even when this is not native to the format. Upmix is generating those enveloping lateral reflections and improving “spinorama” of L-R speakers by compensating for horizontal dispersion shortcomings. Lot of individual filters, lot of deviations to standard Auro - be it volume, delays and some secret sauce in bass management. My room is also acoustically designed to support this type of setup - lot of diffusion, targeted range limited absorption, targeting t20/t30 with constant. until 500Hz and then gradual increase into lower frequencies.
And of course - Planar Bass [Planar Wavefoming] - clearly anchored directionally to front soundstage and filling it, no “collapse” or whatever drama.
Clean Group Delay down to 20Hz, good impulse behavior and uniform first -20dB decay and it offers [in my opinion] superior sound quality - it is making bass reflective and not resonant, which is quality you will not get with resonances and nulls all over the place. Linear rise and decay - LF fundamental frequencies arrive aligned with 2nd/3rd harmonics, that carry spatial information, so yes, you get precisely located bass, you get Billie Eilish and her impressive bass gimmicks.
FWIW - I got most improvements in envelopment when I have taken care of 100-200Hz range properly.
