Tube voltage regulators

Just curious if anyone has rolled in different brands/vintages of tube regulators in your pre/power and what was the sonic differences you heard, if any. 

I have found different rectifiers to change the sonics, but never attempted to roll in different regulator tube brands/vintages before.  Obviously not all tube pre/power use them so this is a bit limited in its scope.  Thoughts?
I've rolled some old stock RCA OD3/VR150 in my preamp but couldn't really tell the difference in sound quality. All in all, there aren't as many options in voltage regulator tubes as there are for signal/rectifier tubes.