Slate Digital has a strong reputation for modeling highly sought-after and nearly unobtainable gear, and their plug-ins generally have an edge on the competition. They pride themselves on capturing every nuance of the original units, especially the nonlinear artifacts behind the unique sound that keeps these pieces in the racks of engineers.
Needless to say, I was excited when Slate announced the release of a new bundle of bus compressors. A bus (or buss) compressor is different than your standard channel compressor in that it's intended to be placed on a stereo master bus or possibly a subgroup of several channels (e.g. drums). A good bus compressor is invaluable for making an entire mix sound cohesive. Slate's Virtual Buss Compressors bundle comes with three of them. They can be used independently or as a rack, where they can be placed serially in any order and are switchable and solo-able in real time.
All three compressors share are a very useful dry/wet knob and high-pass filter. The former comes in handy if you want to dial back the amount of compression without touching the other controls. The latter isn't used to remove low-end but rather to determine how much bass frequencies will effect the threshold. In modern music with loud kicks and basslines, this feature can be helpful to allow those elements to pass through undetected. I found that if the compression became obvious, or if too much bottom was removed, turning the high-pass filter up solved the problem quickly. All plug-ins, including the rack, have a convenient A/B switch for comparison between two sets of settings.
FC-Grey models the SSL4000 bus compressor, which lived in the center of their large mixing consoles. This compressor might be the best-known bus compressor of all time, with a sound you'll find on many hit records. When used properly, it glues mix elements together and gives drums plenty of punch while taming parts that are over the top level-wise. I've used several plug-ins that model this particular compressor, and I can definitely say that the Slate FC-Grey has replaced them as my go-to. When A/Bing it against my former favorite, the FC-Grey mixes popped out a little more and sounded less smeared and even brighter. It works well on subgroups of drums and vocals, too. The Slate team made a slight change to the original design to ensure clearer mids and less bass removal. I don't have the real one to compare it to, but this compressor sounds great every time.
The FC-Red is a virtual version of the Focusrite Red, which is mostly known for its uses in the rock and pop world. The Red is great for adding punch to drums and full mixes when used even subtly. I always test it out on a mix to see if it helps or hurts to have a little touch of the Red on there. One enhancement Slate made to the original design was to incorporate a drive knob that models the unique output transformer of the device. (To get this sort of drive on the original, you would have to push the makeup gain.)
Finally, FC-Mu recreates the sound of classic tube compressors like the Fairchild 670 or the Manley Variable-Mu. This unit has the most warmth and character of the three, and it's useful for sweetening mixes even if you're not using any gain reduction. It definitely brings out some highs and mids, but I also felt like I lost some bottom when using it, even with no compression. This could help you add definition to your kicks, though you'll lose some of the very low frequency content. As with anything, it's all about experimenting with your own material.
The Slate VBC bundle gives access to some very powerful mixing and mastering compressors at a fair price. Using them in a rack and switching their order increases their capabilities further, and their ability to replicate certain vintage pieces makes passing signals through them worth trying even if they are set for zero gain reduction. The differences can be subtle at times, but with proper use, you can take your mix quality to the next level.