After some tinkering and conversations with our own kcress, I have some lessons learned regarding the vfd closed loop:
1) Match the motor to the pump. Obvious, but make sure you understand the motor's power and rpm specs and how they relate to what the pump needs. Mine don't match. It still works but it is not ideal.
2) Be prepared to adjust the carrier frequency on the drive. Mine was very loud with the default setting- there was a loud, chirpy whine from the motor. After playing with the carrier frequency, it is gone. Now, there is just a *barely* audible low hum that disappears when you are more that a few feet from the tank. The fan in the vfd drive unit is louder than the pump. As noted several pages back, I had some noise issues with the return pump, and partially solved them by putting rubber couplings on the plumbing. I did that here, too. It has probably contributed to the quiet operation.
3) Get a high quality TEFC motor, as this is likely a big part of the reason why I got it so quiet, while others have not been able to elininate the vfd whine.
Really, even at this initial experimental level, I am very excited about having this functionality on this tank. I would suggest that anyone else DIY-minded who is putting a big closed loop on a big tank should do this. Or at least experiment with it.