I know this is an old thread, but I just wanted to post this update for anyone who may come across this.
The HOG D3 1st stage works well right out of the box with the OTS Guardian. This setup breathes extremely well at depths below 30ft, but in shallower depths it is a little difficult to breath from not unbearable though. When I get my 1st stage serviced I'm going to have the IP adjusted to exactly 135psi.
Also, I went cheap and bought the XS Scuba QD and it works fine and makes life so much easier. I just get out of the water disconnect the hose and keep my Guardian on until I get to a suitable place to doff it.
A note that I would like to inform first time Guardian users of is that even when the Ambient Breathing Valve open if you breath hard enough, such as in a tiring surface swing to your dive line, you can pull air through the regulator because the ABV can only flow so much air and if you breath hard enough you will begin to draw a vacuum in the mask and since its a positive pressure FFM the 2nd stage will begin to flow to fill the mask back up to ambient pressure. So keep that in mind as you can easily suck down 500psi just the surface if you're breathing heavy, so calm down an don't in a rush.
