Well, I'm more and more curious about this. Just by turning it on, with threshold at 0 and output at 0, so technically no operation at all, the PC4K S-Type Channel Comp puts a boost of 3db on the signal.  The PC76 does the opposite: it cuts 3db by its mere presence. S-Type Bus however, does not exhibit such a behavior.
 
1- Is this normal?
2- If so, why is it like this? Is it related to the modeled hardware?

Spencer
Well, I'm more and more curious about this. Just by turning it on, with threshold at 0 and output at 0, so technically no operation at all, the PC4K S-Type Channel Comp puts a boost of 3db on the signal.  The PC76 does the opposite: it cuts 3db by its mere presence. S-Type Bus however, does not exhibit such a behavior.
 
1- Is this normal?
2- If so, why is it like this? Is it related to the modeled hardware?

To truly take a compressor out of the signal path, the ratio needs to be set to 1.0:1. Remember that this isn't a limiter with a 100% hard knee. 
 
I just tried bypassing/enabling the PC4K S-type channel comp, and the peak value was the same with the threshold/output controls set to 0 and ratio set to 1.0:1.

Ah, I see. Indeed the default 5:0 ratio is causing that behavior. Lowering it to 1 and the peak stays the same. It's weird to me, I've never seen a compressor act on the signal with no threshold at all before; much less actually boost it by raising the ratio only. Some form of integrated auto-makeup gain perhaps. And then there's the 76 doing the opposite, decreasing the gain with just a step up in ratio, until you reach 20 which seems to put the level roughly back where it was on the 4, then the 100 which is the loudest of all; all of this with 0.0 input and output. Quirky stuff if you ask me.

Spencer
Ah, I see. Indeed the default 5:0 ratio is causing that behavior. Lowering it to 1 and the peak stays the same. It's weird to me, I've never seen a compressor act on the signal with no threshold at all before; much less actually boost it by raising the ratio only. Some form of integrated auto-makeup gain perhaps. 

 
No, simply that controls on a compressor interact. The ratio and knee both affect the threshold, unless the knee is hard. So you can set the threshold to 0, but other controls may move it off that value. 

You can see this easily if you call up the Sonitus compressor. Set the threshold to zero and vary the ratio. The only time the ratio won't interact with the threshold is if the knee is hard. The higher the knee setting, the greater the degree of interaction.
 
A simple analogy is an amplifier with an input and output level control. You can set the output for 0, but if you alter the input, you can get more or less level appearing at the output without changing the output setting.
 

And then there's the 76 doing the opposite, decreasing the gain with just a step up in ratio, until you reach 20 which seems to put the level roughly back where it was on the 4, then the 100 which is the loudest of all; all of this with 0.0 input and output. Quirky stuff if you ask me.

 
The "100" is not 100, but the symbol for infinity so it is acting like a limiter instead of a compressor. The PC version models the original quite accurately, which people expect if they've used a real 76. That's considered part of the unit's "charm." I personally don't have a problem if companies want to take liberties with a model and clean up the oddities. But purists would scream...I asked Waves once why they included a switch on their Aphex Aural Exciter module to add the hum and noise of the original, which made no sense to me. They said in listening tests, people who had used the hardware version said the plug-in didn't sound "right" without the added hum and noise. 
 
If you want a "classical" compressor that doesn't try to model a physical unit, use the Sonitus but make sure "Type" is set to "normal" and not "vintage."
 

Most interesting, I had never looked at it this way. Always something new to learn about compressors even after so much time spent with them. Cheers for the pointers.

Spencer
Most interesting, I had never looked at it this way. Always something new to learn about compressors even after so much time spent with them. Cheers for the pointers.

We're all still learning...
 
On a related topic, I thought one of the most brilliant things IK ever did was produce a Mellotron plug-in that let you vary the sound from old, beat-up Mellotron with dirty heads to idealized, perfect Mellotron. I never used Mellotron plug-ins because most of the them emulated the "Mellotron sound." But I DID use the IK one because I preferred the idealized sound. Similarly, I try to design idealized amp sims instead of emulating existing amps. And I turned off the hum/noise switch on the Aphex Aural Exciter...

Anderton
 And I turned off the hum/noise switch on the Aphex Aural Exciter...

Blasphemy !  Big iron, high voltage and cramped spaces more or less guarantees hum.
Well, maybe not at 11, but certainly in my living room.