See what you get for contradicting me, Dundee?
Soul Reaver+4: SoA. Kickass sword usable by Evil Warriors & Bards.
Soul Reaver+6: ToB. Even more sickeningly powerful sword usable by Evil Warriors & Bards.
Unholy Reaver+5: SoA. Kickass sword usable by Anti-Paladins only.
Barbarians are indeed treated as a Fighter kit, according to Shadowkeeper, even though all the documentation says that they're a wholly separate class.
Lord Smite, Cromwell doesn't mention upgrading the Soul Reaver because it's a ToB-only upgrade: One of the required components can only be found in ToB, and therefore only Cespenar can perform the upgrade.
In my game, the Soul Reaver+6 is too powerful: It grants a
bonus of +50% Magic Resistance, instead of the
base 50% MR granted by its relevant component. I hear that some change was made to the MR in a recent update to Weimer's Item Upgrade MOD, I wonder if this was the correction.