How loud is loud, What DB is actually loud?

Frank, Thanks, great link.

If you replace one 8ohm speaker with two 16ohm speakers in parallel then area will double but power to each speaker will half and SPL will stay the same.

Two 8ohm speakers in parallel will receive total of 2x power resulting in 3dB increase in SPL. It is the same as two exactly same subs, that Al mentioned, driven by separate identical amplifiers also resulting in 3dB increase.

It seems logical to me since adding more transducers (area) of the same efficiency without increase in power cannot improve overall efficiency. SPL is increased by 6dB only if power is doubled to each of two speakers (quadrupled power from the amp)

"Play your system in mono, turn of one channel , guarantee you will have more than a 3db loss"

Yes, but not because of the smaller radiating area but because you cut off half of the power.

yes, there is room gain, but since it applies to both speakers it disappears when you'll take the difference - the gain is still 3dB.

I must be missing something - I would thing that if one speaker is creating certain SPL at my sitting position (including room reverberation) second exactly same speaker will double this SPL. Doubling SPL means 3dB increase