I have not heard the Atmasphere and AGD GAN amps but have owned 3 other GAN amps that I liked a lot. Only issue for me was the limited power on the 2 amps I owned. My speakers are not easy to drive. The third amp, the PeachTree GAN400, had enough power but it was a good amp for the price, not great (used $1000). The other GAN amps I had were modded and were very good.
However, the Class A amps I have had were better in power and sometimes sonics. The ugly looking KRELL DUO XD lineup, the latest CODA amps, and even my Schitt Aegir. The KRELL is very similar to the Westminster that was mentioned earlier. Other than the Aegir the Class A amps I have owned were not that hot to the touch.