Both preamps are Foreplay 2 - I can tell from the label on the power transformers.
In the most recent picture that Paul Birkeland (yes there are two Pauls) posted, near the top, he indicated in red the two terminals that should be connected. A green wire goes to the center terminal of that terminal strip (labeled 13) and the one to the left of it (terminal 14) is the other. A short wire between these terminals should be soldered at each end. Technically, this connects the signal ground to the chassis ground - you can see that terminal 13 is bolted to the chassis.
(Whoever does the soldering should be aware that the copper signal ground wire passing through terminal 14 has a really tough plastic coating. Just in case - this calls for 60/40 tin/lead electronic solder with rosin flux; find someone who knows what that means!)
Back in the day (it came out in 1998 if I recall correctly), many customers but not all had a hum problem that was resolved with this modification, and I see from the picture you posted most recently that yours does not have this connection. So there is a good chance that this will resolve the hum prroblem. There are of course many other things that could have caused the hum, so you can't be certain it will work until it is tried.
You do not need a schematic to do this.
I also see from you last photo that this preamp has the stepped attenuators, fancy boutique audiophile capacitors (the large yellow cylinders), and the current sources (the four small circuit boards) - so this is a top-of-the-line version of the Foreplay 2.
