Tobias Growler neck/truss rod adjustment

[Allen_VA]

I'd like to put a touch of relief in the neck of my 5-string Tobias Growler (ca 1997/1998). The instructions that came with the bass indicate a "single truss rod for maximum adjustment". However, removing the cover plate at the body-joint exposes two side-by-side truss rod nuts. I experimented by giving them both a little equal clockwise tweak which seems to be putting BACKBOW into the neck - this is opposite what I'm used to with truss rods (and opposite what the manual says)!

Does anyone know how the TR's work in Growlers from this era? I've turned both nuts CCW until they seem to almost be spinning freely now, and still no upbow in the neck.

Thanks for the assistance!