I brought the SOHA to the last meet at Tulsa. Those that listened liked it. The Stacker II is pretty much nothing like a SOHA. The SOHA had a set operating B+ of around 65V and was based to use a 12AU7 or variant. The Stacker II will have an adjustable voltage from 150-300V, and be able to handle both 6.3V and 12.6V heater requirements. It also will be modular in the sense that you can swap out the front end on it to have a SE front end biased into class A, or a PPA front end (another type of SS headamp that is run back with
a servo--,amy people like this amp).

The Stacker II will support any octal with pinouts identical to the 6SN7, and any B9A (9 pin) tube with either the 12AXX or the 6DJ8 pinout arrangements. That's right, you can roll any of these tubes into the amp along with similar tubes like 6SL7, 6H30, 6N1P, 12BH7, etc.

The Stacker II buffer is self-zeroing and it does use an opamp in the servo. But the servo is not in the signal path and does not affect the audio. Moving the opamp out of the signal path was one of the main goals. And it doesn't use a differential input and there is no NFB. This is one reason that the amp ruthelessly uncovers the sound of the tube that's being used in the front end. This was another goal. I think this design will really show the shortcomings of your source, but we'll have to wait and see.

Although I had some input on the design, the majority of it was done between 2-3 people and I'm basically a guinea pig for the prototypes. Alex has had several designs "stolen", basically he put the design out there to be critiqued and improved, then someone took the schematics and started selling pcb's before a really good version could be done. I am one of only a few people that have a p2p SOHA amp because of that. This time he is holding the schematic close to the vest and will be selling the pcbs through Glass Jar Audio when we've worked all the kinks out. But the design has been put through a lot of simulations and should work.

If anyone is interested in building a headphone amp I have a PCB for the 2nd generation Millet with diamond buffer PCB, a Millet MAX pcb, and a CK2III PCB that I would be willing to bring and sell for what I paid for them. YOu can research the designs on Head-Fi and Headwize.