Got an old VPI PLC......

I had a PLC and upgraded to the SDS. It was a very small difference, although I've heard others state the difference was quite substantial. Most people are very sensitive to speed fluctuations. My guess is that in some environments under some conditions the SDS will perform significantly better than the PLC. In my room the power is already very well conditioned which might have had something to do with the small change I noticed.

Big difference in my case. I assumed it was because the SDS is a regenerator rather than a line conditioner. And two PLCs died on me, which didn't do much for my confidence level. Dave

i just have to jump in here- i bought an aries years ago with this MASSIVE OVERBUILT motor, right? and, it sounded awesome. but i bought the plc anyway, because it was supposed to improve the performance (?). well,
i never noticed anything in particular, but my plc worked fine. then i upgraded to the sds, which makes the motor turn EXACTLY at speed. the improvement was subtle to my ears, plus i had to raise the sds voltage to get the platter to start turning!
and now, the aries has been completely redesigned, which i don't even care for as much as what i have. so, sour grapes or no, i ain't going to touch my turntable anymore. it sounds fine. i can't blame vpi for marching forward with various modifications, but the aries blew me away from the day i bought it, soundwise, looks wise, functioned perfectly. if it ain't broke... anyway, you have to really have a yen for how the sds works, because you may not hear too much, but it is quite sophisticated, and well built.
but if money is an issue, leave it be.