Equivalent power BJT transistor 2sa758 and 2sc898 for Sansui au 6600 Amplifier
What really matters is the I_V output curve, which shows the flatness of the Iout versus Vout for a constant Ibase; these tilted curves are called the Early effect. A high Early voltage (seldom spec'd in transistors) determines the power supply rejection at high frequencies (where the negative feedback has weak ability to control the loudspeaker voltage).

I'd install a small cooling fan in your (upgraded) amplifier; keep those hot transistors 20 degrees cooler and their lifespan should exceed yours and that of the next owner.