What most companies call Auto Bias is actually "Cathode Bias"- A commonly used method that is cheap to make, and often found in guitar amps, but also $20,000 high end audio amps.
An amp that has "70 watts" will typically give you only 40 watts with Cathode Bias. Most of the power goes up in heat through a single huge resistor. These amps run the tubes very hard. They need to so they can squeeze out the power. That means short tube life and failures via "shorts".
Real Auto Bias requires a level of engineering that is very advanced. Only three companies have a real “Auto Bias”. The first was PrimaLuna, which introduced it in 2003. The other is Mystere, a sister company to PrimaLuna. The third is VAC (Valve Amplification Company), who offers it only on their Statement 450 IQ model, selling for $112,000 a pair.
PrimaLuna's Adaptive AutoBias is a true, smart technology. It employs an array of sensors to monitor tubes and make adjustments seamlessly in real time. Adaptive AutoBias is completely passive and NOT in the signal path. The result is low to no maintenance, long tube life, and the lowest possible distortion for amazing sound.