There are many ways a tube amp can keep self-bias on the tubes. It's neither better nor worse than fixed (Manually adjustable) bias. Some of the tradeoffs are that usually one method (fixed bias) allows a little more power output. For fixed bias an amp usually needs a separate bias supply, which adds to cost/size/complexity. The methods of self-bias vary, the most common is cathode-bias where a resistor biases the tube. Other units (BAT Vk60 come to mind) use a more sophisticated method of bias where its more of an auto-regulated active bias voltage than a simple cathode-bias self-regulating system.
both methods sound good, and adjusting bias manually is not a big hassle. Sometimes it's nice to have fixed bias in that it allows changing the operation point of the tube, which is not generally possible with a self- or auto-biasing amp.
-Ed