DHT Sound Better
Directly Heated Triodes (DHT) sound exquisite, period.
Primary reasons DHT tubes sound so good:
1. Linearity
2. Simpler design (removing the cathode and making the heater a cathode)
3. Starving heaters can reduce distortion even further
Why are DHT not made anymore:
A few models are still in production (300B, 211, 845) are the most commonly.