Wow, is that the first Atma-Sphere with an output transformer?
No, nor the first to be shown in public or sold. We make a small 5Watt/channel integrated amplifier that is suitable for desktop, bedroom, headphones or a main system if your speakers have the efficiency.
That one uses EL95s which are a little brother to the 6AQ5, running class A1 ultralinear. The tube is very easy to drive so the voltage amplifier is a differential amplifier using a 12AT7 with a constant current source. The amp can sit on a single sheet of notebook paper with room left over but its built with good parts throughout and an overbuilt power supply. The chassis has its corners welded and ground so it can be polished and chromed. We've been having fun with a variety of finishes and color schemes; blue with black (the transformers be the contrast finish), blue with chrome, black with chrome, chrome with black, grey with red and so on. They are entirely handwired point to point.
It easily keeps up with SETs of the same power; it has a greater amount of usable power owing to greater linearity. It also has wider bandwidth both on the bottom end and the top end (goes out to 100KHz).
This amp is one of the projects we did which aptly demonstrate why SETs are impractical and obsolete.
That's not to say we're not also having fun with SETs. We have a 300b SET project as well. It uses some techniques we use in out OTLs to minimize distortion driving the power tube.
Sounds like the 300B amp is well on track, and I imagine the next version will have current sources in the appropriate positions. I’m curious what your experience with a high-power current source in the output stage turns out to be. It didn’t work for us, but it did work for Allan Wright back when he visited me in the late Nineties.
Constant Current Sources are the key to getting differential amplifiers to perform. A PP output section is often wired as a differential amplifier so a CCS can work really well if output section is class A.