Could you pass the Power voltages check, step 3 of the setup guide?
the DI output (without the preamp) seems to be very low
It is normally 1/12th (-22dB) of the input voltage because of the Jensen transformer (ratio 12:1).
1 VAC at the input gives 80mVAC at the output.
The output level should be the same in the 3 modes.
Still the preamp and the distorion don't seem to work
With the preamp on you should get up to 9 VAC at the output.
If not the case, check U3: value, direction and solder joints.
the ground lift does not seem to do anything
The ground lift can only work in the passive mode.