Tom,
The best way to know for sure where the problem is would be to test your unit on someone else’s boat that has a Humminbird transducer. This can be any of the DualBeam, QuadraBeam or even a Side Imaging unit (but not the 981/987 units).

If you cannot test on someone else’s boat than you should remove the Transducer Switch and plug one of the transducers directly into the unit and test it again to see if you get the same results just to eliminate the switch as a possible source of the problem. If there is a connection issue in the switch it could lead to what looks like reduced sensitivity.

As a last resort you could call the Humminbird Customer Resource folks at 1-800-633-1468 and set it all up to send in your unit and transducers to be tested; but it would be best to try and eliminate what works correctly on your end first if possible.