The first stage is either piston or diaphram, balanced or unbalanced.
The Mk2 is an unbalanced piston. The Mk10, 20 and 25 are balanced pistons. (There are some varients, like the Mk10+ and the aluminum and titanium versions of the 25, as an example)
Seconds are either typically a straight downstream design (R190, R380, G200 and a few others) or balanced poppet designs (S550/600/G200B/G250HP and a few others.)
The balanced poppets tend to breathe more smoothly, simply because they are relatively-speaking less sensitive to IP fluctuations since the design applies IP to both sides of the seat. The spring used to 'load" the seat and keep the poppet closed is thus much less powerful and the delivery of air more smooth, as it only has to make the seal, not work against the IP.
Downstream regs rely on the spring pressure to counteract the IP, and have no balancing feature. They thus are more finicky to tune and also tend to be more critically matched to the IP of the reg they are connected to, and do not breathe as smoothly, although if you take your time you can get pretty close. They also require seat repalcement more often, usually when you can no longer get them to tune reasonably due to the seat becoming excessively engraved.
For all seconds the lever height should be set, basically, right at the diaphram at rest (stage pressurized) For downstream regs the problem with this is that changing spring pressure changes lever height, and vice-versa, so you end up "chasing" the correct settings. Ideally you want the lightest spring pressure that will keep the reg stable, with the lever set just touching the diaphram, that results in the desired breathing resistance (usually 1" or so of H2O; a bit higher for "backups", sometimes a touch lower for primaries)
For balanced poppets the orifice setting controls the lever height, and the balance chamber adjustment controls the cracking pressure. With these the balance adjustment does not typically disturb the lever setting appreciably and vice-versa.
The "newer" SP balanced poppet designs use an "unloading" feature that prevents "engraving" of the LP seat when the reg is not pressurized. Basically, they use the IP to move the poppet and balance chamber into contact with the seat when the reg pressurizes, and it is allowed to float free when there is no pressure. The unfortunate reality of this feature is that the poppet setting is under a pressurized cap, so you either need a special tool to set it or you must pressurize and depressurize the stage every time to want to make a change to it. The good news is that the setting for that is not horribly critical; getting close is good enough. This is even more true for the S600 which has an external adjuster for the balance chamber, allowing you to tune it a bit "loose" and allow the diver to crank it down a bit to stop any freeflow tendencies - something you can't do if there is no external adjustment for it.
For regs with an external adjuster I set them up so that I can get a slight freeflow with the adjuster all the way out; this allows, for most adjustable seconds, a cracking pressure setting of anywhere from very close to zero up to about 2" of H2O, at my option. Just a touch (1/8th to 1/4 turn off the stop) is where I usually dive mine, but if the current is unusually strong I might add a half-turn or so to back off the sensitivity, especially if I "park" my primary for some reason. For regs without an external adjuster I typically set them up for about 1" of H2O on inhalation resistance.
You do not need a magnahelic gauge to make this adjustment - a sink full of water will do just fine. 1" from the top of the diaphram is easily figures on the reg body; simply submerge it slowly, diaphram down, until it starts to flow - there you have it. Its not perfectly precise but then again neither is a mag, and neither are you when you're providing the source of inhalation. Unless you have a full flow-bench setup (very, very few shops or techs do) the two methods are almost identical in accuracy and results.
I've got basically all the common sizes in EPDM - when they're a nickel each you can afford to do that kind of thing, and have 20 of each size for $20 or so. I do have the case wrenches and such that SP likes to have used.
