The point Ibanez brings up in that thread was my first thought as well. It looks like this schem omits a buffer or gain stage at the input
I say this without actually looking for the rest of the schematic, and I’m sure you guys will have this figured out before I get time to investigate. But something to look into if you’re planning on drawing or building this
As Travis pointed out, this layout only features the actual valve section of the preamp.
When checking the schematics, there is indeed a "pre-preamp" consisting of ICs and diodes before the tube section.
I've taken a look and I'm afraid the schem is a bit unclear for my skills. Maybe someone more experienced can add a standalone board for the missing section...