Apeks has offered several model second stages in their XTX series regulators: XTX20, XTX40, XTX50, XTX100, and XTX200 versions in Primary (black), Octo (yellow) and Nitrox (green) livery.
Since 2013† the current Apeks XTX series second stages are all essentially identical except for cosmetics and one minor feature. All the XTX series are a pneumatically-balanced downstream design with a large ergonomic venturi control lever, can be reconfigured to reverse the side of the hose inlet (left or right). They are all Nitrox Ready and all use the same periodic service parts kit.
TLDR; The XTX50 is an all black high-impact plastic second stage with a knob for adjusting the opening effort (aka breathing resistance.) The XTX40 omits the adjustment knob. All the current versions are otherwise identical except for color and offer the same ultra-performance breathing characteristics.
The XTX50 version dress is all black high-impact plastic and the now discontinued XTX200 version adds to the case cover what Apeks describes as an "aesthetically pleasing" polished chrome plated metal decorative ring. The XTX200 "Tungsten" version had a dark matte grey PVD finish on the ring. The also discontinued XTX100 version had a satin chrome finish on the ring.
The XTX40 version has the all black plastic cosmetics but omits the opening effort (aka breathing-resistance) adjustment knob found on the other versions. The now discontinued XTX20 version was a warm water only model that also omitted their patented heat exchanger feature. There are "Octo" dress versions of the XTX20, XTX40 and XTX50 second stages where the color of the case cover & venturi lever is yellow and they are configured with a more compact exhaust tee. Just to be clear, all the XTX second stages regardless of the model and cosmetics are otherwise identical in breathing performance.
There are also "Nitrox" dress versions of the XTX40 and XTX50 with green and yellow cosmetics, but again otherwise no different. The green color is strictly cosmetic and does NOT indicate any internal differences in design, materials or lubricant. Other than the packaging, the green & yellow color second stage is not any more suitable for oxygen service than any other model in the XTX series. All the Apeks XTX second 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.
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 2025 in the US, Apeks markets the DS4 first stage with the XTX40 second stage as the "XTX40 Regulator" (RG114119). Apeks pairs the DST first with the XTX50 second as the "XTX50 Regulator" (RG115128). 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. 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 with suitability for use down to {36°F | 2°C} that make Apeks XTX regulators a popular choice of experienced sport and technical divers worldwide.
Most Apeks dealers know the XTX series first and second stages are all perfectly interchangeable in terms of compatibility and breathing performance. Apeks dealers, including Dive Gear Express, sometimes build up "mix 'n match" regulator packages that depend on customer or dealer preferences for cosmetics and price point. We have even seen cosmetic components swapped between the models, such as an XTX40 'blinged' with a XT200 cover. Just to be clear, describing your Apeks XTX regulator package simply using the Apeks marketing name derived from the primary second stage model does not reliably identify the exact first and second stage versions included with your specific regulator.
For 1st stage differences read Apeks XTX Series First Stage Differences
† In 2013, Apeks updated their XTX series second stage: The venturi lever was redesigned to be larger with a soft touch rubber grip, which makes it easier to locate and use. (A noticeably smaller lever is the easiest way to spot a pre-2013 model. After 2000, the first two digits of the serial number indicate the production year and the second two digits indicate the month.) The new lever also reduced the amount of grit ingress to better operate smoothly. The opening-effort (aka breathing resistance) adjustment knob was improved to make it easier to use. The purge design was improved to better provide two-levels of purge and to better clear debris build-up. The exhaust diaphragm material was improved to further reduce work of breathing. The premium Comfo-bite® mouthpiece was included on all primary versions, the octo versions include the standard mouthpiece. The braided-flex low-pressure regulator hose was included on all models (although the Tek3 set was later switched back to rubber hoses). There were minor changes to badging cosmetics and the ball-joint swivel on the 9/16" end of the LP hose was discontinued.