Pull the grille frame and pull out the 2 JBLs. If they are D140Fs, you're in luck! You can run these in parallel and show your amp 4 ohms. They will be quite loud and have a very nice, "Sunn amp" kind of tone. OIf they are 140-6, they are the 16 ohm versions. If they are D130Fs, they are the more efficiuent, brighter speakers that can't take as much bass as the D140s. If they are D15S speakers, they are basically D130s made for Sunn by JBL.
Let us know what you find.
Tom