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Ah! That's a great question! Ok, this is based on MHO and personal observations...
At "vintage" levels (read "class A" VOX, etc.) around 335V to 345V, biased at 100% they do sound a little more chimey and sweet, but not much balls or dynamics. It's a very specific tone that some guys dig. Then there are amps that run them hard at 390V to 420V (usually just a 10V-20V less on the screens) but with a cooler bias. At which point they lose any signature character and sound like mini 6550's Tight and surprisingly strident and powerful with a lot more LF authority and dynamics, but no chime or noteworthy compression. I like them between 350V to 365V with 10V-20V less on the screens. They do keep some compression like this, not much chime though. But they still sound like power tubes working hard and they have a lot more balls in the LF. My opinion is that the absolute RIGHT voltage depends a lot on the preamp they're amplifying since this, and the power tube character will combine to create all the attributes of the final tone. Whenever the chance presents itself I would suggest starting high and working down with series zeners until you like what you hear for a specific amp. Then install a PT suited to THAT voltage. A PITA I agree. But a lot of us have odd transformers around so it's not usually impossible. el84's are probably my favorite tube. They can sound any way you want them to, from soft and small to big and strong dependent on plate voltage. Some designers have taken advantage of this. Like the Peavey Classic 30 vs. the Classic 50. Same amp, different plate voltage, and very different from one another. It doesn't hurt that the el84 is available in several brands and affordable. They don't tend to last long because they become mechanically unstable (microphonic). My new fave is a Russian tube that is selected for mechanical stability because my own amp is a combo. But three of my customers use heads and there is no need for selected tubes for equal tone. YMMV.
At "vintage" levels (read "class A" VOX, etc.) around 335V to 345V, biased at 100% they do sound a little more chimey and sweet, but not much balls or dynamics. It's a very specific tone that some guys dig. Then there are amps that run them hard at 390V to 420V (usually just a 10V-20V less on the screens) but with a cooler bias. At which point they lose any signature character and sound like mini 6550's Tight and surprisingly strident and powerful with a lot more LF authority and dynamics, but no chime or noteworthy compression. I like them between 350V to 365V with 10V-20V less on the screens. They do keep some compression like this, not much chime though. But they still sound like power tubes working hard and they have a lot more balls in the LF. My opinion is that the absolute RIGHT voltage depends a lot on the preamp they're amplifying since this, and the power tube character will combine to create all the attributes of the final tone. Whenever the chance presents itself I would suggest starting high and working down with series zeners until you like what you hear for a specific amp. Then install a PT suited to THAT voltage. A PITA I agree. But a lot of us have odd transformers around so it's not usually impossible. el84's are probably my favorite tube. They can sound any way you want them to, from soft and small to big and strong dependent on plate voltage. Some designers have taken advantage of this. Like the Peavey Classic 30 vs. the Classic 50. Same amp, different plate voltage, and very different from one another. It doesn't hurt that the el84 is available in several brands and affordable. They don't tend to last long because they become mechanically unstable (microphonic). My new fave is a Russian tube that is selected for mechanical stability because my own amp is a combo. But three of my customers use heads and there is no need for selected tubes for equal tone. YMMV.
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