brf 5-2-2017The short answer is that they are not identical.
...if a rectifier is providing more than adequate DC current demand, how can a different brand of tube sound different if the DC electrical parameters are identical?
Most tube characteristics are specified as nominal or typical values, without a +/- tolerance or max/min specification. (The main exception to that being what are referred to as "absolute maximum ratings," which define the maximum amounts of voltage, current, and power the tube is rated to handle without the likelihood of damage or a significant reduction in longevity). And even in cases where a +/- tolerance may be specified for a given parameter, it will rarely be a narrow one.
So significant variations in electrical parameters can be expected among tubes of the same type that have been produced by different manufacturers, and/or in different eras.
Regarding rectifiers specifically, my suspicion is that what is likely to be the main contributor to sonic differences in many applications is simply differences in the voltage drop across the tube, resulting in differences in the DC voltage that is applied to the circuits the rectifier is powering, especially the tubes that may be in those circuits, resulting in those tubes operating at different points on the curves which define their operating characteristics.
Regards,
-- Al