Thank you. I've compiled a list from posts/internet/blogs (sorry, didn't bother giving credit!) about the features that might not be obvious to all. Please correct or add any info:
The switch next to the VU engages the external side chain for things like ducking. "External In" is just the label for the switch. The "IN" means the external input has been engaged.
SSL threshold knob is often misunderstood: fully clockwise=no compression, fully counterclockwise=full compression. As you turn it counterclockwise it lowers the threshold point (threshold point=point where compression begins) The threshold knob centered at "0" DOES NOT MEAN no compression. Again, fully clockwise means no compression. The historical labels on this knob are really misleading. It's dependant on how hot a signal you have going into the unit (ie: if you don't have a loud signal going in you'll have to turn the threshold counterclockwise more to get it to compress. If you have a louder signal going in you won't have to turn it so much)
"A" is just an auto release, the same as in the original SSL console.
T1, T2 - Tilt filters. High Pass/High shelf on detectors - in other words they undercompress the lows and overcompress the highs. Really nice on drums in particular. Super Side Chain (SSC) Traditional high-end compressors often provide a switchable hi-pass filter, allowing you to help prevent low frequency energy from triggering gain reduction. This is useful, for example, if your mix has a raging kick drum and you don’t want your whole mix being compressed when it strikes. You can high pass up to 100Hz or so and remove the kick’s fundamental frequency (presumably where it presents the most energy) from the compressor’s sidechain, allowing the compressor to respond to material much less than it otherwise would. The picture to the left is a snapshot; notice the strong presence of low frequency material. Now, look at the image below. It represents a hi-pass filter, at 100Hz 3dB down point, at a slope of roughly 12dB per octave. Enabling it would reduce the low frequency content your compressor “sees.” While the SB4000 provides that functionality, it provides two “Tilt” settings, which look more like this: If you’re not a logarithmic person, just imagine that graph as a diagonal line going up from left to right. You’ll notice that while this curve does begin to reduce frequencies below 5kHz, it also makes the sidechain more responsive to frequencies above 5kHz – things like harmonics, cymbals, sibilance, shakers, and lots of other things that begin with an S. The result is a highly transparent option to increase loudness, bring out detail, and shape the dynamics of your mix.
The button to the left of the compressor "IN" affects the internal side chain. It's the internal sidechain mono/stereo button. In mono mode the sidechain detector affects both channels equally, in stereo, they react independently.
Level LED meter: You can monitor the Input (I) or Output (O), but only one channel at a time by switching to Left (L) or Right (R).
Compressor In button engages/bypasses compressor.
