I don't see how a double-ply head would be louder than a single-ply head. It would seem that the double-ply head would be more resistant to moving air, which is, after all, what sound is...
This is weird to me as well...On my DW kit, the G2s give me more of the woody shells' sound than G1s do. On my 1970 Ludwig kit, the opposite is true. I can't explain this. I guess it varies kit to kit. I guess nobody has addressed about how the selection of head (adds/subtracts) the resonance of the shell itself (into/out) of the sound. Does that add to the sound's warmth (thin shells) or brightness (thick shells)? It all factors in, I guess.
Maybe the drumming world has it all backwards--maybe a more freely resonating shell is able to absorb the vibration of the head, putting out its woody sound, but stifling the sound of the head, thereby making the sound quieter. I mean, what are the chances of the head and shell actually resonating in tandem to the exact same frequencies of fundamentals AND harmonic overtones to amplify each other? If there's ANY conflict of frequencies, phase cancellation will occur, which will reduce the overall volume. Hmm...maybe thick shells that don't resonate at all and just act as an air chamber for the heads will become the new desired sound. The new "fad".
"How the head affects the amount of resonance a shell gives off and how that resonance affects the overall sound" would be a great topic for a new thread...