Hi Danny -- No doubt it is due to the relatively low input impedance of your EQ unit. If you electrically "moved" the Tape Monitor switch in the circuit so that the monitor input reflects the same sensitivity as the Aux input represents, then the drive impedance at the Tape Output jack is the same as that presented by the source selected. Therefore, if you have (say) a CD player connected to the Aux input, it should have no problem driving your EQ unit, with likely then no loss of signal. However, the internal phono preamps and the output of the MPX sub-chassis in the 400 are not so capable, so that your EQ unit would likely load those sources down significantly. If you did not move the Tape Monitor switch, then connecting your EQ unit to the old Tape Output circuit will load ALL inputs down equally, since that signal was derived at a different point in the signal chain.
Question: Did you install the little buffer stage that (in part) allows conversion of the old Reverb jacks into new Preamp Output/Power Amp Input jacks? If so, and if your EQ unit is capable of supplying a 2 volts RMS output (almost all such units can), then you could remove the jumpers and connect your EQ unit to those jacks. So modified, the output impedance at the Preamp Output jack is just 100 Ohms, which will more than easily drive your EQ unit, and, it will free up your Tape Monitor input for other sources, too.
I hope this helps!
Dave