FWIW the X650 is not DIR Compliant. The reason being is that you would be unable to take it apart underwater, without tools, to clean any debree in it.
I'm unfamiliar with a "membrane first stage". Can you elaborate on that?
Their are three types of first stages that I'm aware of. They are unbalanced piston, balanced diaphragm, and balanced piston.
An unbalanced piston regulator are typically the bottom of the line regulators, such as a Scubapro MK2. There is absolutely nothing wrong with an unbalanced piston regulator for recreational depths. Many manufacturers make some unbalanced first stages that are tanks and nearly indestructible. That said, they will only serve one purpose well in technical diving and that is to use them on shallow stage bottles and/or decompression bottles. The reason for this is because the deeper you go, the harder they become at breathing from. This can cause some extra task-loading at depth that you really wouldn't need.
Balanced diaphragm regulators are very popular in many realms of diving and are the most common type of regulator around. It uses a rubber diaphragm to direct gas through the first stage. The bad thing about that is the potential of the diaphragm failing, but you don't see that happening to often, and even less likely in regulators that are submitted for regular servicing, as specified by the manufacture. An example of a balanced diaphragm first stage would be the Scubapro MK17, Oceanic CDX, Salvo SR1 and SR2, Apeks DS4, etc. It's also worth noting that most balanced diaphragm regulators are environmentally sealed, which makes them very useful in colder water environments, such as ice diving.
The balanced piston first stages are tanks as well. These use a piston to direct gas through the first stage, but a major difference between them and an unbalanced piston is the pressure on both sides of the piston is equal. This makes it ideal for deeper dives, where having a good, constant, easy breathing regulator helps cut down on the task loading. An example of a balanced piston regulator is the Scubapro MK25.
A Scubapro MK25/S600 combo is an excellent work horse of a regulator. The only thing that I do not like about them is the turret on the MK25.
Unfortunately with your question you are going to get mostly opinions on what the best regulator is. There isn't a good answer. You could go read the numerous articles in scuba diving magazine about good regulators, but most of the companies being tested in those articles paid scuba diving magazine to do so, and it leaves out hundreds of other models out of the article.
I have four regulators. I have two Salvo SR1 first stages with Scubapro G250 second stages for my doubles, an Oceanic DX4 first stage w/ Oceanic GT second stage for my single tank dives, and a Aeres A1 for my decompression tank. If I was to buy another regulator today I would probably buy a Salvo SR1, but not necessarily because it's the best regulator out there, but because of the availability of service parts. That's not to say that I don't think the Salvo regulator isn't a good regulator, it is and I'll continue to do business with them.
