speed stability

The reason for using PSA, SDS or any other regenerator isn't voltage regulation but frequency control. Frequency control allows you to control the speed on tts without that capability, like your Classic. You're going to need a strobe and a disc to measure the speed of your Classic. I'm not familiar with the PSA units, don't know how flexible they are with frequency regulation.

The PS Audio devices are limited in their capacity to alter AC frequency. They were designed to supply regenerated power for any audio gear. However, if your tt is running at correct speed direct off the PS Audio, there may be little to gain by inserting an SDS. Not nothing, but "little" to gain. Unless the PSA is dedicated to the tt, you probably don't want to alter the frequency of its AC output above or below 60Hz, in any case. That might be bad for the other gear.

The spped contol of most TTs is based on teh FREQUENCY ofthe incoming AC. (as others have mentioned)
If you want the exact 60 cycle then yes a PS audio type regenerator may improve the AC frequency control.
But with no testing done it is hard to say if it really does better than the line itself? or better or worse than the average TT speed control.
The most accurate TT speed control are built in ones with a feedback servo.
Like Denon and Technics.

Benjie, SDS & PSA are exactly the same thing, both are digital regenerators! The only difference might be that the SDS will allow finer tuning of frequency adjustment to control speed of synchronous motors. I'm not familiar with the current ones but earlier PS Audion units did have a certain amount of frequency control too.

"By reducing the voltage to the tt motor, the motor produces less vibration and makes a significant improvement in sound reproduction. Hey, we all know that less vibration to a turntable means better sound. The SDS is a truly amazing piece of electronics." This is completely false and is in the same moronic category of using expensive power cords on tt motors and claiming it improves sound quality. There's nothing amazing or revolutionary here, the SDS is just another AC regenerator and at the 2k price there are plenty of more featured industrial units around. Only the interface is audio specific.

Elizabeth, a steady 60hz cycle doesn't guarantee the correct speed of your tt. A frequency controller is a pitch/speed control for synchronous motors and allows one to set the correct frequency to achieve the right pitch for 33/45/78 rpm if your table is setup for it, not every table will give you all 3 speeds. Servo controlled motors, direct or belt driven aren't the most accurate and further servo control has negative impact on sound quality while continuously tweaking the speed.

Lewm, tweaking the frequency slightly above or below 60hz has no effect on electronics, just motors. In general you really shouldn't use a regenerator with your electronics because of the negative impact they have on sound quality. Some manufacturers do advise against using any regenerator with switching power supplies, irrespective of frequency control.