It's odd that one would put RCA ends on speaker cable. RCA's would suggest they are feeding a line-level input on some device somewhere.
1) You mean banana plugs and not RCA's.
2) He did use RCA's and the cables go right to the speaker(s) (and the guy doesn't know what he's doing)
3) They are RCA's but they are not speaker cables (ie, the are line level going to the input of the distributed audio system)
You said he replaced the unit - so let's assume it is number 3. The RCA's lead to a L/R line-level input on the new audio system. Does this system have an outboard volume control somewhere?
your receiver? If so, this is drawing of the rear.
I see two ways to get analog out of the receiver to your other system. Your receiver does not appear to have a "Zone 2", which would be the common means for doing this. Your manual shows diagrams using both outputs to a recording device.
This note was found below one of the sections about recording.
If this were my setup, the first thing I'd do would be to hook up an MP3 player or something to your RCA's (3.5mm male headphone to 2xRCA female). See if/how it works. From there you can see if there's a way to get the audio source you want to the system using your receiver. The note above is important though. None of your digital sources will send audio out via analog. Some sources maye have digital and analog, and you can probably make that work by assigning them to different input selections in the receiver. One input for regular playback, another input for playback over your "zone 2".