If you're getting low-level hum, that is constant level, and does not change in loudness even if you adjust the gain of the preamp, then your power transformer may be interacting with the output transformer of the X-12 pre.
Try any of these solutions, or combination of these solutions.
1. Move the power transformer as far away as possible from the preamp board.
2. While listening to the low-level hum, rotate the toroid power transformer a few degrees until you find the spot where the hum just disappears. The "sweet spot" is probably just a 2-5 degrees of rotation, (i.e. very narrow). A little bit to the left, or to the right, and you'll hear the low level hum.
Once you find the sweet spot, tighten the bolt of the toroid transformer to fix it's location permanently.