The gain is more than good for a 4.5mV cartridge. Something else must be wrong. Measure Ohmic load on phono input with power off and DC mV with power on. No TT cables attached when measuring.
Your bottom tube doesnt bias. You should have 0,7 something if theres a 1N4148 down there with current passing. Measure your voltages in different points like anodes of all tubes. Are the diodes the right way (cathode down)?