I have done every permutation and combination of ways to build this 300 PP circuit. I originally started with high quality CCS and then went to LC coupling, which sounded more natural and musical to my ear, and then finally went to full transformer coupling. As was stated above, this was impossible with off the shelf iron from even a very well know major European manufacturer. Those ITs rang like a bell at 15K and up. It took several iterations of custom design work in collaboration with Dave Geren at Cinemag to get it right. Dave's easily make it down below 20 Hz and out past 30 KHz with no oscillation and no loading network needed. We all have our taste, and all versions of the amp were wonderful, but there is magic in the final version that simply wasn't there with CCS or LC coupling. A lack of solid state coloration and wonderful tonality and presentation of instruments to my ear. The piano sounds like a piano, not a nice rendition of one.
We are in production after two years of prototyping. Sounds like @atmasphere is early on their journey with a PP 300b. Certainly far enough along to demonstrate a prototype at a show, much as we did in Seattle last summer. It is undoubtedly a different topology than ours and I wish them well. I am sure they will get their version to where they want it to be as we have with ours. They will sound different of course due to the preferences of the designers.