Here are some measurements I did with the 4.51 version AGC using the XG3 as a signal generator. For each of the three signal levels from the XG3 -- -107, -73, and -33 dBm, I used the 10 dB attenuator and 10 dB preamp to get 9 different signal levels.
As THR increases, the effect of SLP decreases. At THR=20, SLP is essential 15 for all values of SLP (I tested SLP=0, 7, and 15). This is the upper curve on the figure below. As THR decreases, SLP begins to take affect. At THR=15, SLP values between 0 and 7 are the same while between 7 and 15 an effect is seen. At THR=8 the full range of SLP is effective.
Changing SLP behavior as a function of THR has been done to protect the user's ears.
Figure 1. K3 firmware version 4.51 AGC. RF gain is fully clockwise.