Also, it is a four channel amp. Just to be clear, there are two left channels (not an "entire left channel" as long as it is not in a bridged mode).
I'd triple check (I know, we all know we checked right the first time, but myself, I can get confused about channels), by having only one speaker connected. Front Left is a good place to start. With only a known good speaker connected to Front Left, I would rotate each control, push or turn every switch, putting things back where they were afterwards, listening to see if there's any change. Sometimes that can clear it up too.
But I agree with all the above comments. "All original" is not exactly a selling point with old stuff (unless it is collected for some kind of museum), it simply means there is more likely to be problems down the road. Well, that and it also means no one has done a hack repair/restore on it (that would be a huge selling point as long as the buyer is going to effect restoration before serious use and wants to have someone they trust do the job).