Hi Daneel,
The thing you've missed in your theory is that a 15/18 driver moves way less than a 10/12 to produce the same volume.
Also if you check the parameters of 15/18" driver against 10/12" ones you'll find that 'generally' the bigger drivers will produce more dB than the smaller ones for a given watt.(easier to move?)
More generalising would say that the larger speakers would give less distortion at a given spl than a smaller driver aswell.
On the movies/music Q.....
Movies need large spl's in the lowest registers, music doesn't..
If you look at sub designs then the most musical ones are -usually/subjectively- ib's, horns, or sealed boxes ( and di-poles if you've got lots of drivers going..)
Many people equate 'tightness' with group delay (large in vented boxes, smaller in sealed)
I feel most people on movies would rather have the spl there with a higher group delay, than have lower volumes and 'tighter bass' - this is why svs stuff is doing well - well designed ported subs that give a good, deep response at the right spl.
I believe a pair of sealed 12's would sound better than an vented sub for music, but lack the output for movies..
As a footnote, you'll notice a few people here choosing svs over the servo 15's - this makes sense as I believe the servo15 limited the output at 98dB @ 20Hz, meaning the 'less accurate' svs's actually attained nearer reference levels, but with increased group delay, poss. distortion. - unhearable with a film soundtrack, but probably hearable on music....
For the info on the servo15's 98Db@20Hz see the footnote here:
http://www.stereophile.com/loudspeakerreviews/955/index1.html
Makes you wonder about the 'deep and flat - real bass blah blah blah' claims that have been made on this forum for ages does'nt it?
Hmmm
Cheers
Rob