Does the following hold true with all "Good" boxes such as Acadians etc.?

With my new Gabbanelli, I can press the air valve in fully and play at the same time. I get all notes, even bass and treble together, with only a small loss in volume and no change in tone at all. I couldn't do this with my Hohner 114. Press in the air valve with the Hohner and you loose all kind of sound quality.

This results in the ability to get more air in the bellows without any pause in playing. In other words, my accordion can talk and breath at the same time!

By the way, I use the heel of my left hand to press in the air valve. Seems like the easiest way. My left thumb just stays on the back of the bass keyboard.

Your observations can't be used to judge the quality of an accordion until you first consider the size of the air hole and valve opening. Also other variables are involved

Interesting observation. I never thought about that... I suppose it would indicate some form of air tightness quality. And yes, it is normal to use your air valve button with your left palm, not your thumb.

After further experience, I'm not sure my observations are entirely accurate. The Gabbanelli is much more efficient in the amount of bellow movement required to play it and the reed respose is quicker. The real advantage seems to be the ability to quickly take in more air on a pull note without affecting the tone quality and it doesn't require a full depression of the air hole, only a quick press of the button. An interesting subtlety of technique in playing.

All of this is probably just the process of getting to know a new box and the small variables each one has.