Sounds like your upper motor stack needs some servicing.
Everyone wants more bass. Is your bass normal? That's the question. There are some electronics parts that can contribute to weaker bass. Those would need to be checked or replaced. You can measure the output of the organ at different frequencies to see if any are weaker than others. Play in the middle of the manual for example and compare that output with the low part of the manual. If there is lower output at lower frequencies you may have a weak or bad bypass capacitor on the output tube.
If not, the Leslie may have amplifier problems. Voltages should be checked to see if it is running correctly. Coupling and filter caps may need to be replaced.