I did look into the two transistors that you recommended, however I don't understand why these are better substitutions than others. If you can explain the reasoning, it will certainly help me learn what is important and what isn't when finding a substitute transistor. I understand transistor theory and function, but I don't know the impact of every data point and the values specified on the transistor data sheet.
That depends on how the transistor is being applied so I would need to see the schematics or have a very detailed description of the circuit. I took a quick look at the Marantz 2270 schematics and did not find any 2SA493s. If it is used as part of a differential amplifier, then both transistors should be changed for best performance.
For small signal audio amplifiers in low voltage gain circuits, (1) there is not much variation between different transistors as long as they have the needed ratings and are roughly the same size. A modern replacement for the 2SA493 is the BC560 but watch out for the pinout which is different.
2SA493: (2-5B package ECB Pinout)
Vceo 50V Ic 50mA Pc 200mW
hfe 120 to 400 at 6V and 2mA
Noise Specified
Ft 100MHz Unverified
Cc 10 pF Unverified
BC560: (TO-92 package CBE Pinout)
Vceo 45V Ic 100mA Pc 500mW
hfe 110 to 800 at 5V and 2mA
Noise Specified
Ft 150MHz typ at 5V and 10mA
Cob 6pF max at 10V
I did not find detailed specifications for the 2SA493 but here is what I am looking at. The maximum collector current specification is not too much larger or smaller, both are specified for noise meaning they are intended for low noise applications, and the hfe specifications are both given at 2mA meaning they are intended to operate at that collector current.
Another one which would be good is the 2N5086/2N5087:
2N5086/2N5087: (TO-92 EBC Pinout)
Vceo 50V Ic 100mA Pc 625mW
hfe 150/250 min at 5V and 1mA (specified at 100uA, 1mA, and 10mA)
Noise Specified
Ft 40MHz min at 5V and 0.5mA
Cob 4pF max at 5V
I disagree with others here about the importance of transistor hfe. It is not likely to matter in this case but I would need to see the schematic to be sure.
(1) The output capacitance and reverse transfer capacitance become important in high voltage gain circuits like the VAS stage in an audio amplifier.