Just eyeballing the schematic, it looks to me like the source impedance for a filter replacing R27 would be about 400 ohms. I have no idea how to calculate the output impedance made up of the bifilar winding on T2 and it is likely to be different based on frequency. The best bet in this circumstance would be to assume a low impedance for the output of the filter transformer and probably no transformer would be needed for the input (the Q21 side). The two effects if it is a wrong assumption will be too low gain and a tilted passband. Too low gain is probably not a problem since we are trying to take some out. Tilted passband can be checked by sweeping a tone input from 300 - 2700 Hz and watching the output power. Variation more than 3 dB is probably unacceptable and both USB and LSB should be checked.
WA8TOD
