The LED is what kinda blew my mind when I saw the kit. I never encountered it in the Guitar tube amp world. (Guitar amps are more mid-fi, than hi-fi) So, the current should kinda go both ways, but a diode is a one way deal....crazy. I guess the cathode is negative, but still, the signal is AC. It's hard to picture in my mind. So, I know AC and DC can exist on the same circuit, but I can't picture it in my mind yet. I mean, the AC can "ride" on top of DC. And I guess that's what "elevating" the ground is. I'm still learning.
And then add that it is the bias of the tube and
I've tried Fixed Bias on power tubes. (applying voltage/current to the input) I've tried grid leak bias, where the cathode is ground and the input grid leak resistor is megs. Also, and most commonly, I've tried cathode bias on preamps where you put a resistor to ground on the cathode. ("bottom" of the tube) But a diode? a one way faucet.... I had to buy the Crack after seeing that... Also my HD650s had no bass, I mean...that too....
I mean, I wouldn't call the Crack 12AU7 a preamp, I know the cathode follower is more a buffer or unity gain. You still need a good voltage signal in.
With a resistor vs LED , I'm sure you can hook it up to a swtich to "pepsi challenge" it. And I've thought of doing it. It's just the Crack sounds good, And I'd need oodles of time to do it in a classy way. Good luck and report back....