SIGNA S2 AUX Only comes up momentarily

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With analog stereo connected, activating AUX button only audio comes up just momentarily. Could this be an audio level problem not being high enough?

I have an audio analog equalizer 10 band per channel. I have my own Dolby decoder model which I have used with an audio receiver to allow headset monitoring. The TV digital out feeds this decoder which goes into an amplifier that has worked reliably. Feeding this decoder analog output to the AUX analog S2 input does not stay activated.

I have a hearing impairment in sensing high frequency consonents so I'd like to run the TV digital output through my digital to analog decoder, through the equalizer and into the S2 AUX input.

Any suggestions as to why the AUX input selection will not stay up?

A quick aside. Where is the S2 serial number located. I have the Polk carton but have misplaced the dealer warranty paper work.