This chart, courtesy B.Gavin, shows an AutoTuba loaded with an MCM 55-2421 eight. Half-space, outdoors, 2.83v/1m. For comparison, the blue trace is a JBL GTO 1002D ten in a sealed cab, also half-space 2.83v/1m.
A 100 Hz or lower crossover should be used to tame the AT's rising response above 100 Hz.