Speed Box for turntable

My turntable is VPI Junior (upgraded versions) with alphason tonearm and Denon DL-103 cartridge. This set up is used for years. I would like anyone who are VPI fans and have the use of VPI Speed box or other brand's speed box to tell me what is the difference in sonic performance before and after the introduction of the said speed box.
Bernardwong, I can't speak to the Junior, but I can as to both the VPI 19 MkIII and the VPI Scoutmaster. With either the Walker Audio motor controller or the VPI SDS motor controller, these turntables benefit as follows: the background gets blacker (the silence between the notes), imaging is more stable, and instruments take on a greater solidity and greater apparent dimensionality. The improvements are material and, depending on the resolution ability of the remainder of your system, these improvements either are immediately noticeable or are noticeable after you've lived with the motor controller in place for a week or so and then remove it. However, there may be greater improvements to be gained by upgrading your turntable in other ways before adding a motor controller.
Have also read about (but have not actually verified myself) that upgrading the AC cord connecting to the SDS results in sonic benefits. It would seem then that upgrading the AC cord wired into the motor would also yield improvement, although that one is hardwired & so isn't nearly as easy to change as an IEC cord would be.
Bernardwong, I have had great results running my HW-19 with the SDS. Given the age of my table I took the approach of upgrading the platter, arm, and cartridge, and when I wanted that final "push over the edge" (as Nigel Tufnel would say) I purchased the SDS. I can't say it made my table sound like a brand new, SOTA, $10k+ system, but it did make the noticeable improvements noted by Rushton above.
I would not worry too much about putting an aftermarket power cord from the wall to the SDS. Its function is to condition/clean/filter/stablize...whatever you want to call it...the power it outputs to the motor.
Dlwask, your suggestion not to worry too much about an aftermarket power cord from the wall to the motor controller makes sense and is very sane advice. But in the spirit of full disclosure, I will offer that the choice of power cord *can* make a sonic difference in some systems. In my system with the higher level Walker Audio motor controller and the Walker Proscenium turntable, this is the case. It's nuts, but we can hear the difference. In this crazy hobby of ours, this is just another example of the truism: "everything makes a difference." Cheers.
Rushton, point taken and noted. My technical side always looks to find the answer or reasoning behind things, only to be proven wrong with a listening test. I can't tell you how many times I am just left scratching my head in disbelief when something really should not improve sonic quality, but it somehow does. I've learned this too many times before and sometimes I still wrestle in keeping an open mind. Walker 'table, you lucky dog!
Totally agree with you about being left scratching my head in disbelief on many of these issues, Dlwask. That is one of the never-ending mysteries for me of this hobby. :-)

Yes, the Walker Proscenium tt is a treat. If you're ever in the Philadelphia area I'd enjoy having you over here to share some music. Same to other Audiogoners.
I have an old VPI PLC (third one and I got it used) and it diffently makes a differernce. Not only that but they lower thje voltage automaticly which lowers motor vibrations and adds years to your motor. Only thing is a VPI SDS is $1000 and the Walker is $1600. Not exactly chump champ! I do plan to get the SDS someday....