I don't know the exact value Kyle, but I suspect it does not exceed its original purchase price. JBL was and remains better known for its speakers. At that time, Harman Kardon's Citation series would have been the top line electronics available from the group. The JBL integrated products were likely a derivative of the same design group (in other words, well done) and manufactured to supply JBL franchisees with an electronics product of good quality that didn't require opening a second franchise. Notice that the control knobs Dan posted in his photo are identical to those used on a contemporary HK preamp, the Citation 11. This suggests that the same engineers designed these products and sourced common parts for the JBL amplifiers. But I suspect the Citation products carry a premium over the JBL amps mainly because, as I mentioned, JBL was never heavily in the consumer electronics game. What you have is a rarity that will remind you of your grandfather every time you look at it. Congrats!
Check the knobs on this preamp; same bits!
Comment: If one played the preamp in this photo with the eq pots in the position shown, the noise would have been intolerable! How do I know? I owned one back then.