Our usual response is that it's worth whatever someone will pay for it, but I understand that you are in a somewhat different position than the usual person asking.
First thing is that the serial numbers don't correlate well with manufacture date, so are of little use in determining age. So, what color is the face plate of the Concert Bass amp? Is it black and silver, or all black? The black and silver is older than the black face. I remember the original black and silver in 1974, but I don't know for sure if that was the first year or not. I'm sure that someone else can tell you what the first year of the black face was.
The going rate can change significantly over time. The best way I know of to determine what Sunns are going for at any given time is to go onto ebay and see what they've sold for recently. Not the asking price, but the selling price. the down side of this is that, if no one has sold one recently, you won't find a recent price.
Speculation: I have seen Concert Bass amps go for under $100 to over $200. Condition matters, as does demand. Several years ago, I paid $400 for a 200S cabinet, which is an older and more desirable 2x15 cabinet than the 215S. I'd say that you're unlikely to get that much for your 215S, or, really anywhere near that much. I hope I'm wrong. Last I looked, which was, admittedly some time ago, foot switches went from $35 to $50, which is why I don't have one.
A quick glance at ebay, and I see a silver face Concert Bass priced at $250 did not sell. Likewise, a black face did not sell at $300, so you can reasonably expect to get less than those prices, unless your items are in extremely good condition *and* you find a person willing to pay a premium for that. Good luck.
