Looks like it's just pre-Fender, mid 80's. Nice preamp, kind of advanced for '85ish feature set, looks to be pretty versatile. I would have expected a hard date on that inspection sticker.
Maybe a Concert series based power section, 200 watts into a 4 ohm load?
Won't find many around, Sunn was pretty much on auto pilot in that era, erratic low volume production.
As far as what it's worth, I'd say whatever you are willing to pay and the seller will take. It's not a collectable, sought after, or even cult following Sunn model, not one of the "prestige" models that bring big bucks, which is not to say it's crap, it might be a kick ass bass combo.
Do you like the way it sounds? I personally wouldn't consider parting with 4 bills to have it, but I would say if you think it sounds like 400 of your dollars or better buy it; but be willing to make it a keeper, because with the far more popular and well known Concert series amps only averaging 2-325ish bucks, the market for $400 SB series amps is pretty much non existent.
