Presumably the physical (materials/assembly) differences between tubes might also have an audible impact, but I don't care why they sound different. I've done comparative listening among a half dozen different rectifier tubes and the blue Sophia 274B and 1950's U52 CV575 5U4G are in a league of their own at least in the Modwright PS 9.0.There has to be more to the “sound” of rectifier tubes than just the different output voltages. Let me give two illustrations. First, different brands of the same tube can sound very different, and consistently so—-a GE 5u4GB sounds different from a Sylvania 5u4GB and from an RCA 5u4GB despite all of them having the same output voltage. Second, I have a linestage with a regulated B+ voltage—-whether the rectifier outputs 350 or 390 volts the B+ is held tightly at 275 volts, yet different rectifiers sound different.