I agree. $600 isn't crazy high these days for an SX-980, but it is a little high. I would try to talk the seller down to $500. By the way, the 980 is a truly excellent receiver. You will be very happy with it.
Personally, I think the Sylvania JAN 6SN7WGTA is an underrated tune. It's nicely balanced, warm without being gooey, extended FR in both directions. I've been using them in my Cary SLP-05 for a couple of years and have no desire to replace them.
I have to agree. 1999 is definitely not vintage. 1989...well...that's stretching it to me, although it was 27 years ago now. So we probably have to allow it. What's sad is that when the 90's stuff is old enough to be considered vintage, we who are into 70's era hi-fi will need to come up with another name for it