The published spec for his cab is stated as "usable down to 30 Hz"... which of course doesn't mean you should automatically try!
The 30Hz is nothing more then marketing B.S., the -10dB point is 32Hz, what that means is that you need 10 times the power to make the speaker output the same SPL at 32Hz as it would in normal range of operation, I sure as *beep* wouldn't call that usable!!!
You got the thread correctly except for the gain structure which is not the problem but rather the output voltage of the mixer/pre. The thing is that we want to try everything we can before recomending to buy new gear.
Did you also change the HP filter to 24dB/Octave L/R?