Hopefully you can get a part number off the IC on the back of the module before buying used. Some of these are kits, some of these are factory assembled. The best one is the "A" grade which has the lowest THD from the datasheet. Then there are the grades of the VCA IC used in the build. The expensive one gets calibrated after the build. The cheaper one had no trimmers no calibration needed. There were L-1 VCA kits where the builder had the option of different chips and different grades. Long story short, you might rethink your initial question. They all have their uses and they all can make a signal sound better or worse depending on the patch. Lopass gates are another option and tend to impart lots of color. There are some VCA's that are super cheap sounding like a Mackie mixer channel. There is the Plankton VCA that is based on the Korg Nutube if you want a tube VCA, or Metasonix if you want more extreme tube. Then you have VCA's like the Verbos ATC v1 and v2 that are designed to overdrive. There are some VCA's that are particularly clean and impart no color or sound on a signal e.g. Those devices don't make everything sound better in every circumstance, they make certain things sound better in certain circumstance. If you have a solid understanding of why FET, tube, optical, or VCA compressors impart a particular sound or color (or lack of sound or color) on an audio signal and why those effects are desirable in certain applications, then you should be able to go to modulargrid and review the catalog of VCA options. I'm thinking there might be some really "high end" VCAs out there that sound like "class A fets" or tubes and that take sounds to the next level. If anyone was going to mention ryo apertures, i just added 2 more to my dream rig for a total of 4 as a hifi nut, i don't get why every other conversation here isn't about sonics. i'm starting to think that a lot of uninspiring demos i see on youtube might sometimes be due to clinical VCAs. i'm looking for something that sounds warm & tubey without having to resort to actual tubes to suppliment my dream rack. VCAs that SOUND analog and that could make even a screechy roland 303 sound almost cuddly. sweet, juicy, lush, and larger than life etc. i'm looking for some really good character VCAs now. What i'd like to see now is everyone weighing in on their favorite sounding VCAs. i swapped to an 8x VCA someone liked and added some vactrols that were mentioned because i like them for percussion. i just spent time soaking the info in the "best clean VCAs" thread which prompted me to drop the intellijel quads i planned as a few people seemed to claim they sound "clinical". it seems i'm in a minority here when it comes to sonics as most discussions seems to be more about features. the point of my dream rig is to have the best sonics possible. I come to modular synthesis NOT for noodling, but purely for sonics.