Otto, I think I might know what your issue is:

Are your speakers are set to "large", and your sub is set to "LFE only"?

If so, and you try to make level changes to the sub during PCM playback, the trim levels can't have any impact on the sub level, because technically there is no sub!

However, when you do combine attenuation levels to below 15dB, the tone suddenly kicks in despite the fact it should not be there.

Please check this out and let me know.

Best,

Scott