WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Most of those "upgrades" are smoke. They yap about the sound quality of rectifier tubes for instance. They even grade them for sound quality! Arrrrgh.
They recommend changing the 6CA4 for a 6BK4 rectifier. The 6BK4 has a higher plate resistance. I would not do that but have performed these mods for people. The cap changes are like a fashion statement. If you have the money & it makes you feel better - okay. The volume control change is silly, there are others you can use that are just as good.
The SA-5.1 was designed as close to correct as he gets from the factory. The weak spots are solder joints from the tube board to the main PCB. Heater connections in the power supply should be checked.
I used to repair these under warranty so .... believe whoever, but try to use common sense.