The Kraken and the Phoenix-HPR are very similar. The plastic case (of the Kraken) does have a slightly better horn geometry to match the HPR jet a bit better. There is a performance difference, but it is hard to notice by most divers.
If you want to adjust the venturi up to be aggressive, you need to use a DSV mouthpiece with the flow diverter. If the flow diverter is not in place, it is very easy for the flow to go past the diver and out the exhaust.
I have intentionally adjusted the HPR venturi high enough (by blocking most of the bleed air port) to the point that even with the flow diverter in place, every breath induced excess flow that went out the exhaust. The more I inhaled the stronger the amount of bypass/ wasted air.
With the flow diverter in place, the induced flow tends to eventually stop when I exhale into the mouthpiece. But with the traditional curve mouthpiece (and a strong venturi flow) the flow can easily become an uncontrollable roaring free-flow.
In general (with the DSV diverter mouthpiece) it is very safe to totally block one of the side bleed ports. Most divers will be very happy with the performance with one side fully blocked and the other side open.
I tend to partially block the second side bleed port. But, this adjustment is a bit of a trial and error approach. Half of the second orifice blocked is about the most aggressive venturi without encountering flow bypass issues (out the exhaust). I have also found that my settings can be too aggressive for some divers (too high).
If you decide to push the limits and see how high a venturi assist and performance you can squeeze out of a Phoenix-HPR or a Kraken (with the DSV), I would strongly suggest plenty of pool time testing (in all positions) or test dives where some wasted air (out the exhaust) is not a concern.
Note: Some free-flow can be also caused by malfunctions, like a sticking demand valve lever or icing due to freezing moisture on the mechanism, etc. A strong venturi can also compound those malfunctions, but they can also occur totally independent from each other.
Buceador con escafandra autónoma clásica.