Hi - welcome to the forum!
This is an interesting issue - even with very low output pickups, you should get plenty of overdrive by noon on either volume control.
Here are some suggestions:
- Eliminate all the potentially silly stuff; make sure you are plugging a known working guitar straight into the amp using a known working cable; plug into all of the input jacks; etc
- swap the pre amp tubes around to see if the amp's response changes at all
- the breakup in the normal channel doesn't happen until the phase inverter, so I'd suspect a preamp issue like you mentioned (preamp not passing enough gain); TMB channel should generate plenty of OD on its own before hitting PI, which also suggests preamp issue and not PI or power amp.
- Check your voltages - I wonder if your preamp plate voltages (ECC83 pins 1 and 6) are very low:
http://www.byocelectronics.com/board/viewtopic.php?f=49&t=52297- Post clear, in-focus pics of your build to get more eyes on component placement and wiring