On thing I apparently forgot to post about in this build thread was the output balancing transformers I used. The preamp designer didn't balance the output for whatever reason, and just recommended using a 600:600 balancing traffo, so I went with Sowter 3603s. At first Sowter shipped me the wrong model. Had to ship them back (to the UK!) and wait for the correct ones, but they made it worth my while. A friend 3D printed me some custom holders/brackets too which worked perfectly.