You can point your brother to my M5 review at the link below under "customer reviews". Unfortunately, judging from what appears on my phone, all my careful formatting has been wiped out so it may be hard to read.
The OW are no different than the bookshelves with the exception that the holographic effect may be more 2D than 3D. I'm fortunate that I derive a pretty good 3D effect because they flank a bow window. It's a great way to declutter a room.
Incidentally, there is no agreed-upon standard for on-wall or in-wall speakers. The spinorama science doesn't apply. So manufacturers do the best they can by nailing the OW to a section of wall and take curves for half space instead of full space. How manufacturers conduct this test, interpret their results and modify their design largely determines how good the OW sounds in a customer's home. Axiom uses a 4'x4' section of drywall for this test BTW. Axiom did a great job BTW as I've said in many posts.
As for a sub, he won't know what he's missing until he adds one.
https://www.axiomaudio.com/m5-bookshelf-speakers
