...I've got the truss rod wound all the way off...
Do you mean you've turned the t-rod nut to the point that it is loose, or that you have turned it all the way anti-clockwise to the point it has stopped?

I'm under the impression that all PBG kits come with dual action truss rods? A 2-way rod should be able to correct back-bow unless there are other issues at play there.