The only thing that can truly be gained is that at best, while blinded nobody could tell the difference above random chance.
You don't need a control when comparing two competitors. No placebo needed.
"Which one is traditional, ie, "better" which one is non-traditional, ie "shouldn't be as good"- or more generically: can blind listeners recognize Spruce, Mahogany and maple vs "non-musical" woods.
If a control was introduced one might compare each guitar individually vs a pre-war D28 then have the blinded listeners guess which was the Martin and which was the "variable" guitar in each case. But that is not the same question put forth in the above exercise.