Small Footprint SS Power Amps

Look at my StereoTimes website review on the SPL S800 single chassis amplifier. It fits the size you are looking for. It's made in Germany, very reasonably priced, and I made it a 2020 Product of the year because of its beautiful performance. I bought the review piece because I enjoy it so much. In my opinion, it's in another much higher league then the amps you mention in your post.

Teajay

Various Bel Canto models. 

Many small-shop monoblocks using PuriFi or NCore modules would fit your description. Some well-known vendors are Nord in the UK, Apollon in Slovenia, and VTV in the US.
These are class D but I would not rule them out for that reason. I replaced a more expensive AB amp with PuriFi monoblocks and haven't looked back.

Go  by sound not size.

Maybe the AGD GaN amps. There is a thread on them.

BTW - I have 2 AHB2’s hidden in my sons IKEA toy shelf which are 12 inches wide. A perfect fit.