The way you have it set up it works like an ejector nozzle. The angled cut improves suction when the pipe is perpendicular to the airflow and the open side of the angled cut points downstream. It's all dependent on actual pressures as I mentioned before. Say you're vent line is getting 0.5psi less than the pressure in the exhaust. That would be great if the pressure in the exhaust is 0.5psi or less above atmosphere. If the pressure in the exhaust is say 1.0psi above atmosphere, then you are 0.5psi above what the vent would normally see.

You could try putting a water manometer on your fitting to see if you have suction or pressure. Even easier, just mount a length of fuel tubing (that you can see through) on that fitting on the muffler and see if it will pick up water out of jar or if it's blowing bubbles.

The sound will change with the relative distances of the exhaust to the individual manifolds. Just like a cast manifold sounds different that a header on your V8.