Number confusion here we go! 
If you're interested in how to tune the S600, here's how I do it....
You have to remove the "DIVE" emblem from the end of the adjustor knob. Just pry it out - there are two little tabs that you can pull back to make it a bit easier, but at the end of the day it has to come off. Discard it (its worthless really anyway other than as decoration)
Behind there is a hex nut recess. Oh, yeah, all this stuff is plastic - be careful and don't overtighten anything - you can strip it. All seals in here are made with O-rings, not crushing things together.
Ok, attach your inline adjuster and pressurize the stage. Crank the DIVE adjustor (on the stage) all the way out to its stop (there's a plastic circlip inside that prevents you from unscrewing it all the way.)
Now adjust the inline adjustor to set the orifice, while shaking the stage, until it JUST stops rattling. This puts the lever right up at the diaphram. DO NOT go any tighter than this! If you're going to be off, be just a TOUCH loose rather than a bit tight. You DO NOT want to "preload" the lever; that severely hurts the stage's sensititivity.
The stage should not be leaking any air. If it is, you PROBABLY have the lever height too high - turn it down just a tad. If you just start to hear a rattle when well-shaken (while pressurized!) then you're very, very close. (When not pressurized the S600 WILL rattle loudly; that's the "anti-set" mechanism working and is NORMAL.)
Now depressurize the stage and loosen and remove the hex bolt. Behind it you will see an internal slotted screw surrounded by an external slotted collar. Here's where it gets fun. Using a small screwdriver or other flat tool, HOLD THE EXTERNAL collar in place. If the stage leaked significantly, turn IN the internal screw. If it did not, turn OUT the internal screw. Begin with 1/4 turn or so at a time.
Replace the hex nut and repressurize, then re-test. You will have to make SEVERAL tries to get it right. You have it right when it JUST hisses a tiny bit with the user adjustment all the way open to the stop. Turning it in to the point where the resistance goes away (from the collar inside the stage being up against the case), which is about 1/8th turn, should stop the slight freeflow.
It will take several attempts to get it right - trust me on this. You will be swearing by the time you're satisfied at the pain in actually moving that silly internal screw. Keep at it - its worth it to get it right, as you will shortly see.
When its right, your cracking pressure with the knob all the way out should be very, very close to zero, and air delivery should be extremely smooth. For normal diving use you turn in the adjuster just past where its "tight" (from being all the way out); you will have a cracking pressure of about 1/2" of H2O, and the stage will breathe very nicely. If you're in a serious current and need to crank it down some, you can turn in the user adjustment and "tighten" up the inhalation resistance - all the way in will produce about 1-1/2" of H2O on inhalation - not high enough to be unsafe or prevent you from drawing a breath, but enough to stop freeflows. It'll also be enough that you'll notice that the knob is in the wrong place
The venturi adjustment can then be used to "fine tune" air delivery "boosting" at depth. That adjustment is really a matter of user preference.
MOST of these regs when delivered from the factory that I've seen are set too tight, and many of them have the lever hard up against the diaphram as a means of compensation. This won't hurt anything internally, but the reg breathes like crap when tuned that way.
Tuned right its as smooth as anything on the market
The G200B, by the way, breathes just as well but has no "user adjustment." Its nice feature is that the balance chamber adjustment is externally accessible WITHOUT removing a plug (its the hex on the opposite side where the hose comes in; it looks like a plug but its actually the adjustment AND airtube cap, all in one. As a consequence you can set that 2nd stage for optimum performance in about 30 seconds - just set the lever and then adjust the balance chamber until you get the desired inhalation resistance. The only "gotcha" is that because it does not have a user adjustment for current and such on the outside, you need to set it a bit "tighter" than you do the S600 or you can have freeflow problems with it.)
If you're interested in how to tune the S600, here's how I do it....
You have to remove the "DIVE" emblem from the end of the adjustor knob. Just pry it out - there are two little tabs that you can pull back to make it a bit easier, but at the end of the day it has to come off. Discard it (its worthless really anyway other than as decoration)
Behind there is a hex nut recess. Oh, yeah, all this stuff is plastic - be careful and don't overtighten anything - you can strip it. All seals in here are made with O-rings, not crushing things together.
Ok, attach your inline adjuster and pressurize the stage. Crank the DIVE adjustor (on the stage) all the way out to its stop (there's a plastic circlip inside that prevents you from unscrewing it all the way.)
Now adjust the inline adjustor to set the orifice, while shaking the stage, until it JUST stops rattling. This puts the lever right up at the diaphram. DO NOT go any tighter than this! If you're going to be off, be just a TOUCH loose rather than a bit tight. You DO NOT want to "preload" the lever; that severely hurts the stage's sensititivity.
The stage should not be leaking any air. If it is, you PROBABLY have the lever height too high - turn it down just a tad. If you just start to hear a rattle when well-shaken (while pressurized!) then you're very, very close. (When not pressurized the S600 WILL rattle loudly; that's the "anti-set" mechanism working and is NORMAL.)
Now depressurize the stage and loosen and remove the hex bolt. Behind it you will see an internal slotted screw surrounded by an external slotted collar. Here's where it gets fun. Using a small screwdriver or other flat tool, HOLD THE EXTERNAL collar in place. If the stage leaked significantly, turn IN the internal screw. If it did not, turn OUT the internal screw. Begin with 1/4 turn or so at a time.
Replace the hex nut and repressurize, then re-test. You will have to make SEVERAL tries to get it right. You have it right when it JUST hisses a tiny bit with the user adjustment all the way open to the stop. Turning it in to the point where the resistance goes away (from the collar inside the stage being up against the case), which is about 1/8th turn, should stop the slight freeflow.
It will take several attempts to get it right - trust me on this. You will be swearing by the time you're satisfied at the pain in actually moving that silly internal screw. Keep at it - its worth it to get it right, as you will shortly see.
When its right, your cracking pressure with the knob all the way out should be very, very close to zero, and air delivery should be extremely smooth. For normal diving use you turn in the adjuster just past where its "tight" (from being all the way out); you will have a cracking pressure of about 1/2" of H2O, and the stage will breathe very nicely. If you're in a serious current and need to crank it down some, you can turn in the user adjustment and "tighten" up the inhalation resistance - all the way in will produce about 1-1/2" of H2O on inhalation - not high enough to be unsafe or prevent you from drawing a breath, but enough to stop freeflows. It'll also be enough that you'll notice that the knob is in the wrong place
The venturi adjustment can then be used to "fine tune" air delivery "boosting" at depth. That adjustment is really a matter of user preference.
MOST of these regs when delivered from the factory that I've seen are set too tight, and many of them have the lever hard up against the diaphram as a means of compensation. This won't hurt anything internally, but the reg breathes like crap when tuned that way.
Tuned right its as smooth as anything on the market
The G200B, by the way, breathes just as well but has no "user adjustment." Its nice feature is that the balance chamber adjustment is externally accessible WITHOUT removing a plug (its the hex on the opposite side where the hose comes in; it looks like a plug but its actually the adjustment AND airtube cap, all in one. As a consequence you can set that 2nd stage for optimum performance in about 30 seconds - just set the lever and then adjust the balance chamber until you get the desired inhalation resistance. The only "gotcha" is that because it does not have a user adjustment for current and such on the outside, you need to set it a bit "tighter" than you do the S600 or you can have freeflow problems with it.)
