The 1689A is a two-channel mixer. It has very good transformers on both inputs and a transformer on the main out (mono). There is an unbalanced line output for each input. I have successfully used these for stereo applications.

The bass and treble circuits are in the mono main output and will not affect the line outs.
The mic/line switch bypasses the input transformer.
Throughput gain can be as much as 54dB (selected with internal switches).

Maximum output is 18dB. Good gain structure through a 9444A would be with the amplifier's volume controls around 2 o'clock.