When performing any tests on an audio system, An AC millivoltmeter - calibrated in dB - with a range of 30V down to 3mV full scale (80dB range) would be extremely useful. Attach a microphone (electret mic capsules are quite good), and you have a relative sound level meter, even better if you have some way of calibration.
http://sound.westhost.com/project16.htm
It is an additional board that should work with all DAT decks that have a Sony CXD2601 signal processor IC inside.
http://www.solorb.com/dat-heads//pbscms.html
has provision for connecting stereo outputs from four different sources/channels as inputs and only one of them is selected/connected to the output at any one time.
http://www.radioland.net.ua/electronic-circuit-id-28.html
Describes basic audio transformer uses like balanced in/out and signal splitters
http://www.lundahl.se/pdfs/papers/groundin.pdf
You have to get two audio transformers which have 1:1 transformation ratio and greater than 1 kohm impedance.
http://www.epanorama.net/circuits/audio_isolator.html