Thanks to W7LR, I had a chance to measure the AGC of the Kenwood TS590. The graph below shows the audio signal relative to the max audio signal for AGC off and max input signal (1100 uV). For the K3 measurments, the dBV readings from the K3 were used. For the TS850 and TS590, a Tektronix TX3 True RMS meter in the dBmv scale was used. The signal generator used was an HP8654B and the absolute signals were not calibrated. However, relative comparisons can be made.
The K3 AGC threshold (THR=8, SLP=0) is at about 3 uV -- S3 on K3 s-meter. (Blue curve)
The TS850 AGC threshold is about 10 uV -- S5 on K3 s-meter. (Red curve)
The TS590 threshold is about 100 uV -- S9 on K3 s-meter. (Green curve)