Doc B.

Thanks for the kind words. I haven't scrounged up a 421A to try myself yet, but if it works like the 5998 (and probably better?) it should be the way to go. I happen to like Mullards (CV4003/M8136/12AU7) for the input tube and Tele ECC82 can be nice too. The Mullard adds a little more lower midrange flesh to the sound to my ear, and the Tele is a little more towards neutral. Both have a nice clean top end. If someone wants a brighter sound the rather rare Siemens nickel plate ECC82s are very clean and speedy on top.

Thanks for crunching the numbers. It's not a huge difference, but enough to make a situation where one is experiencing a touchy volume control even a little more touchy if a 5998/421A is used. Don't get me wrong, I think they sound great and use a 5998 in my own setup.The 6080s and 6AS7s can vary a bit in actual performance so the measured numbers can vary a bit from the calcs. (5998 seems a little more consistent, but I only have a few samples)

The 421A (and the 5998 ) has a higher mu than the 6080, so the hair trigger volume control thing that happens when you have a high level source or sensitive headphones will get worse. Ditto subbing a higher mu tube for the 12AU7. A lot of folks who are used to SS circuits mention hearing a little noise when the volume control is turned way up with no signal present, like to a setting way louder than one could ever really listen with music playing. That's just the nature of tubes with no feedback, a compromise necessary to get the good stuff that tubes offer. What we seek is to get a dead quiet background at what would be considered a very loud but not damaging listening level. Crack can definitely damage your ears (and your cans if they are reasonably sensitive), the source level is high, and you peg the level control. One could try a shunt attenuator. It gives you the voltage divider for attenuation that you are looking for and allows you to set up the components so that the pot is just a variable resistor that is shunting signal to ground and you can use a high zoot resistor as the series element (that the signal passes through to get to the tube's grid).