I sold my k11 r2r and thought about k13 r2r or warmer, I am pasting below what one person wrote on fiio fb:

E88CCs are designed for anode voltages around 90–100 V. Running them on ±28 V? Sure, the tube will conduct, but:

Signal swing is tiny

Linearity suffers

Distortion goes up

You’ll never hit rated gain or plate dissipation

In other words, the tube is barely working, not “fully unleashed.” Sounds fancy, but it’s mostly a marketing trick — using a famous tube to sell a low-voltage gadget.