Doesn't the Dual Rectifier already have both solid-state and tube rectifiers? In fact, wasn't that the reason it's called Dual Rectifier? Because you can choose between tube and solid-state rectification?
Yes that's right
Originally it was because it has the "switchable rectification" BUT... Later when Mesa released the Triple Rectifier, it became more associated with the number of 5U4 rectifer valves.
I think the Dual recto has 2 rectification modes, as does the triple recto (rather than 3), but the single rectos are solid state rectification only.