Specific transistor for voltage regulating?
Given the flatness of output I_V collector characteristics will affect linearity and power-supply-rejection ability, I'd expect the Early Voltage to be a key parameter for audio power devices.

And the size of the high_current contacts down into the silicon may also matter; this requires slightly larger metallization which requires more silicon. Another way to do this is with additional bond_wires, which definitely increases cost (slightly).

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https://en.wikipedia.org/wiki/Early_effect

A large EARLY VOLTAGE tells the designer that particular bipolar transistor is a more constant current source as the Vce varies.