Apeks has offered several different first stage models in their XTX series regulators: DS4, TEK3, DST, FSR and FST with a variety of port configurations.

Since 2013† the current Apeks XTX series first stages are very similar except for port configurations and cosmetics. All the XTX first stages are over-balanced diaphragm design with an environmental seal. They are all Nitrox Ready and all use the same periodic service parts kit.

TLDR; The DS4 has an asymmetrical port alignment mounted in fixed positions along opposite sides of the body, with one HP port. The TEK3 is based on the DS4 with port alignments oriented on one side for use with doubles. The DST has a symmetrical port alignment with the LP ports on a circular swivel turret, plus two fixed position HP ports along opposite sides of the body. The FSR also has symmetrical port alignment, but the fixed position ports are all mounted at angle to the body. The DST and the FSR have optional 5th LP port upgrade kits available. All the first stages have the same ultra-performance breathing characteristics.

Apeks offers various XTX series first stages each packaged with a different pneumatically-balanced downstream second stage, marketing specific packages using only the name of the primary second stage. For 2023 in the US, Apeks markets the DS4 first stage with the XTX40 second stage as the "XTX40 Regulator" (RG114119) for USD 649.00 retail price. Apeks pairs the DST first with the XTX50 second as the "XTX50 Regulator" (RG115128) for USD 789.00 retail price, and the FSR first with the XTX200 second as the "XTX200 Regulator" (RG113122) for USD 949.00 retail price. Apeks markets the TEK3 (based on the DS4) with the XTX50 second as the "TEK3 Regulator Set" (RP112114) packaged as a mirror image pair of regulators for USD 1779.00 retail price. The nomenclature can be confusing, but keep in mind all the Apeks regulator packages are EN250 certified to a depth of {660 fsw | 200 msw} and have identical extreme performance breathing characteristics that make Apeks XTX regulators a popular choice of experienced sport and technical divers worldwide.

The DS4 has a compact machined body with one standard high pressure 7/16-inch hose port and four standard high-flow low pressure 3/8-inch hose ports in an asymetric fixed orientation on the side. When paired with the XTX40 primary second stage the DS4 body has a polished chrome finish.

The DST has a machined body with two standard high pressure 7/16-inch hose ports, one on each side of the body, and four standard high-flow low pressure 3/8-inch hose ports mounted on a round swivel turret. The DST has an option to add a 5th port on the top of the regulator body. When paired with the XTX50 primary second stage the DST body has a polished chrome finish.

There are also "Nitrox" dress versions of the DS4 and DST with green cosmetics, but otherwise the US versions† are no different. When factory paired with a yellow & green Nitrox second stage, there is a green colored spacer between the DIN fitting and the body. The green color is strictly cosmetic and does NOT indicate any internal differences in design, materials or lubricant. Other than the packaging, the green accent first stage is not any more suitable for oxygen service than any other model in the XTX series. All the Apeks XTX first stages are oxygen clean and oxygen compatible although Apeks has the same maximum 40% oxygen liability disclaimer common to the rest of the recreational diving industry. Dive Gear Express services all Apeks regulators using the same oxygen clean procedures and oxygen compatible supplies.

Based on the DS4, but with different port alignments, the TEK3 it specifically designed for doubles (aka twinsets). Left Hand (LH) and Right Hand (RH) first stage versions are mirror images of each other with ports that are made to route downward to allow for a streamlined configuration that avoids snagging, crossing hoses and restricted access to your valves. When paired with the XTX50 primary second stage the TEK3 body has a polished chrome finish.

The flagship FSR has a beefy forged one-piece body with two standard high pressure 7/16-inch hose ports and four standard high-flow low pressure 3/8-inch hose ports in a symmetric fixed angle orientation that Apeks describes as being optimum for hose routing. The FSR has a removable ARCAP metal alloy high-pressure seat crown. The insulation from the surrounding environment provided by the beefy forged body, together with the replaceable HP seat made of the ARCAP alloy, makes the FSR even more adapted to diving in cold water and contaminated environments. Those FSR stages manufactured after September 2012 have an option to add a 5th port on end of the regulator body. When paired with the XTX200 second stage the FSR body has a polished chrome finish. The now discontinued XTX Tungsten Regulator package included a 5-port FSR first stage in a matte dark grey PVD finish.

The now discontinued FST had a forged one-piece body with two standard high pressure 7/16-inch hose ports and four standard high-flow low pressure 3/8-inch hose ports in a symmetric fixed angle orientation. Those FST stages manufactured after September 2012 had an option to add a 5th port on end of the regulator body. Paired with the also discontinued XTX100 second stage, the FST body had a satin chrome finish.

For 2nd stage differences read Apeks XTX Series Second Stage Differences

† In 2013, Apeks refreshed their XTX series first stages: The non-standard 1/2-inch low-pressure port was discontinued. (Apeks actually had been deprecating inclusion of the oversized port much earlier.) Upgrade kits were offered that added a "5th port" to the end of DST and FSR & FST first stages. However, the 5th port upgrade kit for the DST and the 5th port upgrade kit for the FSR & FST are different. The FST also received a minor improvement to the internal flow path in order to support the 5th port option. With the refresh, the first stages received minor cosmetic changes to their body shapes and to the outline of the badge, which is the easiest way to spot pre-2013 versions. After 2000, the first two digits of the serial number indicate the production year and the second two digits indicate the month.

† In the European Union, SCUBA valves and first stages intended for use with any breathing gas containing more than 22% oxygen must use the M26x2 connector mandated by EN 144-3:2003 "Outlet Connections For Diving Gases Nitrox And Oxygen". The purpose of this incompatible connection is to force the dedication of cylinders, regulators and fill stations to Nitrox use. The M26x2 connectors on Apeks first stages in the EU have a handwheel with alternating yellow and green stripes. The M26x2 outlet and connector for Nitrox and oxygen is very rarely seen outside the EU. The US version Apeks first stages differ from EU versions in that nitrox and oxygen first stage SCUBA regulators have the typical 300-Bar DIN connector (G 5/8" BSP) with a plain polished chrome metal handwheel.