How to get audio out on a Denon 591

If, when the receiver is on, the TV speakers work then you can extract the audio from the HDMI but that would only work for video sources (not the CD or tuner) and only when the TV is on.
You could use one of these
http/www.crutchfield.com/p_142LOC80/Scosche-LOC80.html
with a speaker selector switch to feed a house system. The selector would let you turn off the front speakers connected to the receiver. If you use the receiver for surround sound with a sub then you would have to set the receiver to stereo, full range front to get bass into the home system and turn off the sub.. The volume control of the receiver would also affect the volume of the house system.
There is no great solution to use that receiver. If you can run your sources directly to the preamp inputs that would be much easier.
Consider adding a Sonos Connect to your A-bus system. If would add a lot of music sources and the phone app is great for control. You might not need the receiver at all since the internet radio apps are better than over the air.
You could eventually transition from the Abus (which is not supported by any manufacturers anymore) to a Sonos system.