Please see my response to another similar question.
The SDS lets you set the speed by changing the frequency of the AC supplied to the turntable motor. For example, you can switch between 33 and 45 rpm with the push of a button. As Sidssp so aptly described in his earlier posting, the SDS also facilitates fine speed adjustment in case the table is running slightly fast or slow.
Your PS Audio dealer is selling for his own benefit, not yours, & is either misinformed or is fabricating "facts".
I have personal experience with the SDS. The truth is that the SDS is more than a high stability waveform regenenator with a variable frequency output. It is also a standalone motor controller, comparable to only the more expensive Walker unit. These are true motor controllers because after they start out at full output voltage, they then sense the diminishing of motor startup current & consequently reduce their output voltage to a minimum required for stable motor operation. The result of that voltage reduction is (sonically) quieter / more stable motor operation due to reduction of magnetic saturation.
PS Audio is not designed with this critical feature - your PS dealer is selling BS.
Wanted to update, purchased a SDS and found it superior to the P600. The turntable speed was close with the P600 at 60Hz but at 59.76Hz it was dead on for 331/3. I'm not sure if that small difference accounts for the improvement or the line conditioning of the SDS. The timber and the pitch were improved and basically it just sounded more musical.