Troubleshooting the active mixer by circuit simulator

Trying to find out the cause of the poor performance of the active mixer, I switched from soldering desk to circuit simulator and it looks like a little good hints is found after some tweaking with the resistors values. Assuming all transistors are identical, if the base current of the lower side transistor and that of the upper side transistor are having a ratio of 2:1, the output would make a lower distortion and higher output amplitude. However, how to verify the base current of the transistors on the actual mixer board? I don't know. May be I should apply the bias resistor value from simulator to the actual board.