If you want to run just a single source (cable TV for example) then run the RCA outputs from your cable TV tuner directly into the 170s.
If you want to match your current audio source then you will need to run ANALOG audio (red/white RCA cables) between all of your sources and your A/V receiver (I think - I'm not sure on this, so test it first!)...
Then, you should be able to plug the 170s into the VCR 1 OUTPUT jacks (red/white) on the back of your 5990.
The current source feeding the TV will also always feed the 170s and you can just turn them on/off as you want to and mute the audio, or turn it way down when you don't want to hear it.
I do this with my wireless headphones and it works very well. Okay, I do it very differently, but it amounts to the same thing. The audio from my headphones matches my TV and is ALWAYS on and ALWAYS available for me to listen to. In my setup, I definitely have to run the analog (red/white RCA cables) to the headphones as well as the digital audio which is used for surround sound. It works really well and I've been doing it that way for years.