A bit of extra info on orthogonal balancing.
With the balance weights mounted close to one side of each wheel the system balances out the radial unbalance component of each wheel. However it takes no account of unbalance in the axial direction i.e. the wobble if the wheel is not properly mounted square to the shaft.
This is not normally a problem but, to avoid possible trouble, be sure that the wheel mounting washers are accurately square to the shaft and also dress true both the wheel periphery AND the wheel sides before balancing.
In my initial experiments I loaded the voice coil with a
5/8" dia x 5/16" slug. This gave me a lot of trouble with the 100Hz torque variation signal from the single phase motor. This is because I had accidentally made the "accelerometer" resonant at 100Hz! I doubled the slug thickness, this dropped the resonant frequency to about 75Hz and I had no further trouble.Your speaker may have a different suspension stiffness so it's worth checking the loaded resonant frequency. The easiest way is to lightly tap the slug and to use the 'scope to monitor the damped sinusoid that results.
Jim