Assuming the cathode follower has a large load resistor of which only a small fraction is used for auto-biasing then its input impedance ought to be enormous i.e. it shouldn’t be loading the '81 significantly at all. So you should see a voltage gain of pretty much mu, shouldn’t you ? That should be about 60 for the '81 at 4.5mA and Va~180V (what you’d need to get 4.5mA with a 430ohm bias resistor). Can you measure the signal voltage at the grid ?
I’m sure the modelling will have allowed for the interaction between the cascode’s relatively high output impedance and the 6.2k filter impedance and also for the filter losses at whatever frequency you’re measuring (1kHz ?).