I recently bought an Advantest 3361, I have no idea how similar they may be.
The 3361 uses an internal 30 Mhz calibration signal for self test, and the first thing to do might be to check that the 30 Mhz calibration signal is correct.
The calibration signal is switched right at the input connector with a relay.
If you have a tracking generator, that can also be very useful for comparing the 0dbm tracking generator output with the 0dbm input reference level.
There are multiple gain adjustments in the IF amplifier. In fact when you switch through the various bandwidth selections between 30Hz and 1Mhz, every IF filter has its own individual gain potentiometer.
There are very detailed instructions in the service manual for doing all this if you can locate the appropriate service manual on the internet.