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I am UK based, all my regs are Apeks, mostly XTX200's and XTX50's, I just ended up with loads of XTX40 which ended up using for all my stage/deco cylinders. But good idea on changing them to XTX50's.since it says you are in the UK, I would stay with Apeks, maybe consider upgrading the XTX40's to the 50's when they go in for their next service since it's a simple kit to install but there is nothing out there that is appreciably better than what you have to warrant changing from a performance basis. You are also supporting local manufacturing in the UK which is good.
If you were not in the UK then there would be other reasons that I would argue may merit changing, but to support a local manufacturing operation when you have cheap access to parts and service over there, there isn't really a good reason to go looking elsewhere
I tend to use MK5s (I have the elusive MK5 DIN fittings) and 109s for stages, and I use a MK2 and 108 for my O2 reg. The MK2 has 3 o-rings exposed to high pressure, none dynamic. Two of them are on the DIN fitting (tank and retainer seal) and the other is on the single HP port. The seat is a very small plug of plastic and SP makes an O2 compatible seat. I don’t think there’s a better 1st stage for high FO2 mixes.