I changed the connection like you did.Make sure that the two drive lines are not earthed, the drive is differential and
if one side is earthed you only get one half the driving signal.
How can we check if a set of sensors are best for each other?Try different pairs until you get the closest received amplitudes.
I will take a scope to check the CRO signal. ( can send scope pictures)
That would be very helpfull.
You only need low resolution photos, these would be really usefull:
(Make sure you have the voltage levels in the photos)
Output from 1st stage with example program CRO.
Output from 2nd stage of the amplifier with example program CRO.
Output from the LPF with example program CRO.
Output from the envelop detector with example program CRO.
Output from the zero crossing detector with example program CRO.