[davesdeville]

The subwoofer looks like this one (I don't know what 4 Ohms or 8 Ohms is or what is the difference between the two) http://http://www.bestbuy.com/site/olspage.jsp?id=1069302879271&skuId=6252459&type=product&productCategoryId=cat03081

Ohms are very important, fyi. An ohm is a measurement of resistance, so an 8 ohm sub is twice as hard for an amp to push power through compared with a 4 ohm, so an amp can only push half as much power through an 8 ohm as compared with a 4 ohm. So the lower the ohm rating, the easier an amp can push power through it, but amps get hotter as they push more power and this is why you don't see many amps that are stable at 1 ohm or lower.

Another point is that if you hook 2 4 ohm subs in serial (in a series), they will have an 8 ohm load. If you hook 2 4 ohm subs in paralell, they will have a 2 ohm load. This is useful if you have a mono amp that's stable to 2 ohms, and 2 4 ohm subs (which is a very common setup, in fact I have this setup) or something similar.

 

[davesdeville]

I agree with you, your post mentiones the correct basics. However, the quoted statement is not accurate. It is not necessarily half as much power when you go from 4 to 8, as is not necessarily twice as much if you go from 4 to 2.

Generally it is. If an amp's rated at 400w at 4ohm, it will run 800w at 2ohm. The exceptions to this generalization are when an amp would overheat or maybe distort if it were pushing 800w at 2ohm, so the manufacturer keeps the power lower.

Subwoofer size by itself does not make any difference in the amount of sound you can hear.
Explain me how a difference in diameter can make a difference in dB.
And don't say is a matter of personal taste. This is physics and audio basics.

You're right, but keep in mind that majority of the time 15s are rated to handle more power than 10s.