I've been evaluating an SDS with my Scout for a few weeks. I thought I'd share my experience for the benefit of others.
When I first plugged it in and calibrated the speeds, I was disappointed. Where was the life-changing difference I'd been reading about? OK, the 33/45 switch is handy, running the motor at 72V seemed to eliminate the previous low-level hum, and speed stability was slightly improved on sustained piano notes, but the cost seemed hard to justify. But I kept listening.
Fast forward... Familiar records are more fun. I'm muttering "gosh, that sounds good" more often. It was time for The Experiment. Out came the London H.M.S.Pinafore box--the good one with the color libretto from 1960. First, play side 2 with the SDS; lovely, alive, better than I've ever heard it. Next, plug the Scout straight into the wall and repeat: it's flatter, mushier, a bit murky, like a good recording, but still a recording. Of course, it wasn't worse than pre-SDS, but now I appreciate the fuss. Finally, back to the SDS to confirm the findings.
Here's what I heard with the SDS: the background is much quieter, deep bass is stronger yet with a more complete overtone structure, both micro- and macrodynamics are significantly improved, notes from all instruments are sweeter and more authentic, the soundstage is larger (especially depth), the ambient decay is longer and more linear, instruments have more air around them, and timing is more precise. Best of all, transparency is raised to a new level, allowing the music to be more involving emotionally, and closer to a live performance. It's more obvious, e.g., when singers turn their heads or pass in front of each other. My system was not deficient in any of these areas, but the SDS has sharpened its strengths. The SDS has taken a sizable bite out of the distance between my Scout and a friend's HRX.
I've been told that the hum reduction is likely a benefit of eliminating powerline noise, which makes AC motors do odd things that cause vibration. Otherwise, I suspect the improvements stem from the reduction of tiny nonlinearities in pulley speed, many times per second; these aren't gross enough to be heard as pitch change, but they spoil timing accuracy and result in sour musical tones.
So, why did the SDS take several weeks to impress me? I suppose it, like any other new amplifier, needs a break-in period before settling down to consistent, linear operation.
I'm keeping it.