VPI 300 RPM vs 600 RPM Better?

@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.

I strongly suspect that the amount of motor vibration varies quite a bit based on the position of the magnets in the motor which I understand is not very precise.

Small changes in the phase of the motor drive makes quite a bit of difference in how smoothly the motor runs.

You can probably do this by tweaking the phase shifting cap value but the easiest way for me is to use a computer to generate the drive signal using an inexpensive frequency generating software.

I went back to the 600 RPM

Be careful about putting the motor pod on a compliant material such as sorbothane. As with most things in audio there is always a compromise involved; pluses/minuses. For best sound, the motor should be as mechanically grounded as possible; IOW, not be able to move. Sorbothane allows it to move as the belt tension and it's own internal vibrations affect it's stability. You have to weigh the very real advantage of a lower noise floor against the advantages of absolutely stable mounting. Movement of the motor in relation of the platter has a significant effect on speed stability, dynamics, and introduces it's own kind of "noise" (image blurring). Choose your poison. I found that a thin cork sheet under the motor pod was the best compromise for me.

I've got my motor pod sitting on top of this EAR product:

http://www.allsealsinc.com/1b.html

It has dampening foam seated in between 2 aluminum plates. Very hard and stable and still has some dampening and isolation properties.