How much does tube brand REALLY affect sound in terms of rectifier tubes?

Voltage sag and compression aren’t the only cause of changes in the sound of a component's presentation.  Mainly, that affects it’s dynamics. That’s why I said, "sometimes", that will improve with a rectifier change. The short answer to your question is, "yes". BUT- there’s much more involved than just a, "different brand" involved. Most of us that have enjoyed various aural improvements through rectifier upgrades, have been using higher quality, NOS pieces, manufactured decades ago. Given that relatively few newer components have been designed with valve rectifiers(and/or regulators), there’s not been much demand and little motivation to produce great ones, these past 30 years or so.

Thanks rodman9999.  Yes, very few new tube amplifiers are employing tube rectifiers in their circuit because of voltage demands.  I still see quite a few preamps that use tube rectifiers as the voltage demand is within the tube's capabilities.  I am just having a hard time understanding that 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?  In any event, I going to try again, some different tube brands in my preamp. 

There are lots of things we can't understand , don't let the "facts"  trump 
 your ears .

There are a few companies now manufacturing tubes, that have tried to produce valves, copying the internal construction/design parameters of some of the most popular, from decades back. Mainly, what they’ve succeeded in duplicating(capitalizing on) is the marque only(ie: Telefunken/Mullard/Gold Lion/Tung Sol), as none genuinely perform like the originals. The time involved in hand construction, attention to detail and(in many cases) materials used, are no longer either profitable or(perhaps) available.Try some authentic, older, well reviewed valves(as mentioned above). Worst that can happen is you’ll have to turn them, should they not turn you on. It may cost a few bucks to experiment, but- what doesn’t, in the audio affliction/addiction? Our, "rewards" are SO subjective! Happy listening!

brf 5-2-2017
...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?

The short answer is that they are not 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