I think you’ll have to go by ear, unless you have some fancy test equipment. A lot of vintage Tannoys only had light internal damping applied on top of fine tuned lightly braced cabinets, and then there’s the question of drive unit attachment...
In any case it won’t hurt to read the experiences of a fellow enthusiast, Volker.
http://truefi.blogspot.com/2014/01/a-tannoy-15-enclosure-for-enthusiast_29.html?m=1
In any case it won’t hurt to read the experiences of a fellow enthusiast, Volker.
http://truefi.blogspot.com/2014/01/a-tannoy-15-enclosure-for-enthusiast_29.html?m=1