Hmmmm, did you coat the backside? I would say that is more the cause OR... Try this, move the guitar away from your body. See if that makes any difference.
I have rarely had any issues with deadening of volume or tone when I completely cover the box with finish. I even did a while PUNCH box with shellac in the French Polish method. Sounded fine to me.
Sometimes having the guitar back up against your body will cause the sound to deaden.
I have not bothered to seal any of mine, but I did have a sound change just by adding a large round piezo disc inside the lid once. I took it out and the good sound came back. Thanks for the heads up, I'm not going to bother with sealer now.