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Low Notes and Cabs

Below the frequency range the cab is designed for, the speaker cone no longer transfers energy to the air efficiently. The cone motion is no longer limited by air pressure, and mechanical damage may occur even at power levels well below the cab's nominal power rating.

As @lug said, you can't really hear the fundamental frequency of those very low notes anyway; your ears hear the overtones. So use a high-pass filter to cut out frequencies below the cab tuning.
 
Be careful. As you go lower and louder you start to get speaker distortion overtones which are at the same frequency as regular string overtones. Pumping up the level will make your speaker start flapping around without making much volume.

Try 160hz at nicely audible level then 80hz then 40hz without changing the volume knob.