VPI 300 RPM vs 600 RPM Better?

The only purpose a motor serves is to maintain the correct speed of the turn table. It is out of the signal path. So I suggest you check the TT speed to make sure it is correct and stable.

Another way a motor can affect sound is the motor vibration transmitted to the turn table. That is the basics for the 300rpm motor upgrade because it is spinning at lower speed and vibrates less than the 600rpm one. So if the motor isn't quiet enough, you might want put a drop of motor oil in to quiet it down. The user manual has details on what oil to use and where to put it.

I think you will get better result with this approach than relying on some mysterious break in process.

The 300rpm motor is quieter.

Try this: take the belt off the motor so that the platter won't move, lower the stylus onto the platter, switch on you amp and turn up the volume. Now switch on the motor and you should hear hum.

The hum will come and go as you switch on and off the motor. The hum isn't electrical, it is from the vibration of the motor transmitted through the table and shooting back up to the platter from the cone feet.

If you try this experiment with both motors, you will find that the hum from the 300rpm is much much lower.

@Tzh21y, The 300rpm motor definitely should be a lot quieter. That is the whole point of the upgrade. If it isn't, I think there is something very wrong with the motor you bought. I suggest you contact VPI and see if you can have it exchanged.