Obie_fl is right. I suspect you are bringing the sub trim down because you feel the bass in all channel stereo is too strong. However, all channel stereo is a bad mode to adjust sub level in because your asking the subwoofer to reproduce the L/R bass signal once for the mains, once for the center and once for each pair of surrounds.

Therefore, I suspect you are trimming the sub level well below what would be equal SPL's between your subwoofer and the other channels.

At this point, the sub trim is too low when the tone is generated. Once you move beyond -15dB of trim, the sub level jumps dramatically.

Best,

Scott