... So, if a regulator has a balanced first and second stage, diver adjustable air flow, large enough second stage (for air volume), never diving in water below 40 degrees, is a name brand, THEN why spend huge money on a regulator??? If they are available for $300.00 then why spend much, much more??????
I suggest your requirements may be excessive for the application:
Balanced First Stage:
In my view, balanced diaphragm first stages have gotten a little too good at low tank pressures. A distracted diver can breath a bottle down to 100 PSI before noticing abnormal resistance. Most modern unbalanced pistons produce perceptible resistance at 300-400 PSI, yet still allow sucking the bottle down to 100 PSI if really necessary. We used unbalanced piston first stage regulators below 600' on HeO2 in the early 1970s with acceptable performance. They are far better today.
300 PSI remaining will inflate a BC just fine. 100 PSI will fill it, but there won’t be much left to inhale. If you plan to never fall below 500 PSI, you will never see any difference.
Adjustable Air Flow Second Stage (as opposed to a venture adjustment to prevent free-flow on an Octopus)
I see no reason for an adjustable air flow second stage unless you are using surface supplied air/gas, or your first stage can't hold its intermediate pressure closer than +/- 25 PSI. Kirby-Morgan Corporation developed the "dial-a-breath" in the 1960s. They modified a US Divers (AquaLung) second stage on their full face mask for commercial divers. Air was supplied from the surface by a hose from low pressure air compressors, maybe putting out 150-185 PSI. Naturally, the regulator would free-flow at that pressure when on the surface.
The diver would increase the spring pressure on the second stage seat so it would stop free-flowing and breathe easily on the surface. The diver would back-off the dial-a-breath on descent as the differential pressure reduces. For example, 160 PSI – 29.4 PSI at 66' = 130.6 PSI over bottom pressure. A first stage of a Scuba regulator maintains in the neighborhood of 130 PSI over bottom pressure so there is no need for the diver to adjust the second stage. A properly serviced second stage should deliver optimum performance without diver adjustment. The exception here is if you run a hose from the surface connected to a compressor or large gas bank for decompression. Adjustment is needed in that case just like the surface supplied diver.
So why did the Dial-a-Breath migrate to Scuba regulator? Tell me you don’t see the cool factor here!
I recently suggested a procedure in another forum section for selecting suitable regulators that you may find useful:
http://www.scubaboard.com/forums/5027324-post18.html
You can get new well-proven name-brand regulator for less than $170 that I would not hesitate to use under any sport diving conditions that is more than 5° above freezing temperature of the water.