On increased sound level with multiple speakers
I went looking for the answer to this in Yamaha's "Sound Reinforcement Handbook". According to that source, combining identical speakers will give you an increase of between 3db and 6db SPL. If I read and understood that passage correctly, the SPL gain will vary with frequency and with listener's position relative to the speaker.
So yes, combining speakers will give a higher SPL. But wait...
If you're running a tube amp and adjusting impedance to properly match the combined speaker load, then the output of your amp will be be the same. Put the same amount of power into two speakers instead of one, and each speaker will see 3db less power.
So you lose 3db power by running two speakers, but gain between 3db and 6db SPL. Therefore, the total SPL gain is between 0db (none) and 3db (a noticeable volume increase).
A lot of what people hear as they add speakers is (a) a real increase in low-frequency response due to the greater cone area, and (b) an apparent increase in on-axis high-frequency response due to the correlation effects of multiple drivers.
So the way I read it, multiple speakers will sound louder, all other things being equal. I don't think this is inconsistent with the laws of physics. We're not talking about total power radiated. To measure that, you'd have to combine the sound pressure received in a complete sphere surrounding the speaker. That's valid, but it sure isn't the way we hear. What we're really talking about is the power reaching the listener's ear. And for a listener who is close to being on-axis with the speakers' centerlines, that does increase with the addition of more speakers.
(Just to confuse the issue, let's toss transistor power amps into the mix. If you lower the speaker impedance, transistor amps will (up to their design limits, at least) drive more power into the load. So if you run two 8 ohm speakers in parallel, the transistor amp will put out about 3db extra power. That increase adds to the apparent 3db to 6db increase due to the extra speaker.)