Intriguing DIY single-driver full-range transmission line speaker.

If these interest you, head over to that other forum DIYaudio and check out the full range sub forum. There are a number of designs available for free in any budget for any driver, a few available as flat-packs to purchase as well. Discussions about modifications to drivers and how to fine tune enclosures after they are built. No paid promotions necessary, the designs are "open source" and tweaked by multiple builders until they are dialed in for each popular driver available on the market.

What that YouTuber is experiencing applies to most well designed full range speaker/enclosures. The first time I heard the set of Frugelhorns I built I was blown away. It's a reality check that the best parts of music are not at the high and low end of the frequency range but right in the middle, and when you get that right, without a bunch of electronics and filters mucking up the signal, the listening experience is amazing.