Are both Left and right channels low in level or just one ?
The transmitter has individual opamps for each channel but the reciever shares an op amp. Unlikely to develop two faults at the same time so if both channels are low then most likely the receiver.
Can you test with the units connect just by a short patch cord. Failing that if you contact me off list and arrange to send them to me I will test them out and see what the problem is.