NOS vs new manufacture tubes sound different mainly because the oxides that we used and method to manufacture were toxic, and thus are manufactured anywhere but the USA. Also, many old oxide formulas were lost as the companies were sold for scrap. Last, the quality of mica mined is no longer available so there are trace leakages going on.
New manufacture may not sound like an original of the same type, but then again there can be vast differences even among same NOS brand year of manufacture.