I don't often differ with
@tbone1004 , but I think the Hollis 500se is a
very nice reg. Its valve mechanism is servo like the Poseidon, but of a different design. The servo itself is a downstream valve, so you'll get a warning if your first stage IP starts to creep. The gas flow is a little less abrupt when the servo triggers the main valve. Like the XStream, it is pretty much position-independent, due to the proximity of the exhaust valve.
It is much drier than its Omega predecessor. I love mine, though I ended up adding a right angle connector and 48" hose to the second stage instead of a too-stiff stock hose, to route it up under my arm.
The plastic case, like the Omega, needs gentle handling when attaching hoses. Hoses must be attached to the valve BEFORE attaching that assembly to the case, to avoid damaging the plastic valve holder with wrench torque on the hose connection. But the case design is nice and compact, and with my 90° swivel was very comfortable.
That said, Hollis support is going thru a rough patch right now, since the purchase by Huish. I think it'll end up better than before the purchase, but that's not guaranteed.
The DC7 first stage is a mixed bag. Great balance and crisp performance. Very lightweight. But hose routing is one choice only, unless you add swivels. Mounting it upside down makes it worse.
If you have service and parts, I wouldn't recommend against it. But for sheer confidence that things will be supported 15 years from now, Poseidon and Apeks might be better choices. Tough call, especially if you like your previous Hollis regs.