The only way you can get an idea of market value is to research the market. The best resource is eBay completed auctions, however there is so little Sunn PA gear listed it's often hard to find a completed auction for comparison.
Set the trend for SPL-8815's; what did you pay for yours? Better yet, how much would you take for them?
There has to be a number you would let them go for, and if you aren't interested in selling, what difference does their resale value make?
Figure out what you would take for them and list them in the classified section, or put them on eBay. That will get you the actual market value quickly!
This isn't a "troll for bids" site, it isn't set up for that like eBay is.