I can see a couple of ways of doing this. Not being a 990 owner, I can't speak directly to how these ideas would work, but...

On my 1050, if I select an audio only source, like CD, whatever video I was previously watching is still being fed to my TV. So you could try hooking up the analog audio out from your cable box to an unused audio only input and then take that to your TV via the REC OUT. So you could be watching your cable normally, then switch to say, the tape input, and the video is still being passed to your TV, but the audio has now switched over to analog stereo being fed out from the REC OUT jacks to your TV.

Alternatively, you could feed the analog audio outputs directly to the TV. Of course, then they will always be feeding to the TV so you'll have to mute the TV when you're listening to the speakers and mute the speakers when you're listening to the TV.

Somebody else may have a better idea, perhaps someone who actually owns a 990, but those are the ways I would try it.