One thing that the manual says is...
Replacement for output and driver transistors, if necessary, must be made from the same beta group as the original type.
....what exactly does this mean and how do I make sure that the replacements, if I decide to replace, are appropriate??
If you look at the service manual page that shows the driver board components, you'll see:
Q419,425 2SA968(O) or (Y)
Q422,424 2SC2238(O) or (Y)
Here is the datasheet for the 2SC2238
When you look at it you'll notice that there's 3 variants basic, 2238A, and 2238B. These differences are based on the voltage rating of the transistor (160V, 180V, and 200V respectively) Your 775 only needs the basic rating of 160V, however, you can use the other two variants as they are rated for higher voltage. Next, scroll to page 2 of the datasheet. At the bottom of that page you'll see a section that lists hFE classifications. Those are the "beta" groups you're wondering about. Basically the gain of the transistors. When you replace them, you want them from the same grouping so they'll be matched for the circuit. Since the 2SA968 is the complimentary pair of the C2238, if you need to replace them you need transistors from the same group. So, when you look at these Qs in your unit, they should all have an "O" on them, or all have a "Y" on them following the transistor number that's printed on the case.
If you need to replace them, the recommended modern replacements is
MJE15032G for the 2SC2238 and MJE15033G for the 2SA968