Amit, there are two schools of thought. One is the kind who like boomy sound. These people usually go for large drivers and excessive power. The other is the kind for whom the sub should merge with the rest of the system.
thats not correct. There are tons and tons of commercial subs that use 10" and 12" drivers and are boomy and one note subs. you can make a 8" sub boomy:lol: e.g. just put it in a much smaller than needed cabinet. it will alter the curves and put a boom in it. Usually a boomy sub is the result of a bad design (many times intentionally) in the driver or the cabinet or choosing the wrong sub for your room.
you go for a sub depending upon the size of the room. if you are in a very small room, go for a 8" or a 10" sub. if you are in a larger room (e.g. typical hall+open kitchen+dining etc), you need a 12 or a 15 or an 18". One of the best subs in the world is tc sounds lms 5400 and is an 18" monster. Exodus maestorm 18" or 21" are also damn good subs. These need large cabinets and large rooms to operate properly. A smaller room will have a lot of room gain in the lower frequencies. if u put a large sub in a small room, it will sound boomy due to very high room gain at lower end.
It also depends upon what kind of music you listen to. e.g. classical that has pipe organ can go down to 10-15hz. your usual music will go down to about 30-40hz. If you want a sub to go down to a very low level at a reasonable level, you need a large sub and large cabinets (i.e. without putting equalizers in the electronics, which most of the manufacturers do, but then the head room gets lesser and lesser).
In the old days, sub manufacturers have used large drivers for, shall we say, more sound? Not anymore. Most mass market subs use drivers between 8 to 12 inches. The internal electronics, cabinet construction, bracing, crossover, and amplifier go a long way in deciding the quality of the sub.
thats true. hardly anyone is making commercial subs larger than 12".
Yeah, for more spl and it goes lower.
If you are looking for musical sub, the story is different. You should be looking for a sub that literally disappears and makes you wonder if it is working at all. A sub that merges so well with the front speakers that will make you wonder where the sound is coming from. A sub that is tight and that kills a sound instantaneously. A sub that makes you identify each thud in a roll of drums individually. A sub that enables you to identify a tabla from a mridangam, from a electronic drum easily.
Such subs usually have a 10 or 12 inch driver with about 400-500 watts of amplification.
You can have a 21 incher if you have the room size and can have a large enough (read fridge:lol

cabinet (without your wife killing you:lol

and still get a very tight sound. These large subs though are only possible with DIY as no manufacturer makes these as the dealers dont have the room to host the large ones. Not only these need more floor space, these also need large rooms to play properly.