hmm always thought when you pushed in the sub would come back very slowly how my old subs were. Got a Diamond 10 in sealed enclosure and it comes up pretty quicky. If it comes up slow its sealed and if it comes up quick it has a leak.
actually it owudl be the other way around. Sicne when you push in on the cone you are compressing the air inside the box.
so if it was perfectly sealed sicne you pressurize the box to an extent by pushing in the cone it iwll return like I said above (quick or quicker)
If it had a leak when you push on the sub it woudl force air out of the hole that is causing it not to be sealed. depending on how large the hole is then it might rise slowly since you would have to wait for the air to return to the inside of the box.
The box can be sealed good and still rise back quickly, but usually the box is a little too big if it does that. Most sealed enclosures I have done for myself or customers that were made to the recommended size do cause the sub to rise slowly.