Box building and choosing subs

The advantages of a dvc over svc sub is the wiring options.  For instance...  A subwoofer that has 2 4 ohm voice coils can be either wired in series for an 8 ohm load, in parallel for a 2 ohm load, or you can even only wire one voice coil for a 4 ohm load.  You must also take into account that the subs power handling will be reduced by about half if you use one voice coil.

There is an advantage of a 2 ohm load over a 4 ohm load, but it all depends on what you want.  An amplifier that can "receive" a 2 ohm load will put out about two times the power in a 2 ohm load when compared to a 4 ohm load.  It usually isn't exactly twice as much; it's usually less.  This can only help you though if your amp is 2 ohm stable.  The problem is...  The lower the ohm load you give your amp, the worse the sound quality is.  Guessing that your a daily listener, this won't really matter.  You probably won't even be able to tell the difference between a 2 ohm's or 4 ohm's sound quality.  If you're in a sound quality competition, then that could be important. 

The difference between a 10 inch and 12 inch sub if usually loudness and bass extension.  A 12 inch sub is generally louder than its 10 inch counterpart.  This is due to the increase in surface area.  Larger subs can also gain a little low end extension.  This doesn't always occur, but a 12 inch sub might be able to play lower than a 10 inch.  Everything I just said though only applies if the 10 inch and 12 inc h subs are the same brand and manufacturer.  Things vary greatly between different lines and brands of subs.

The whole "good," "better," and "best" sub can't really be answered.  There are a lot of things to take into consideration.  You have space requirements, money limitations, and most notibaly, hearing differences.   The best advice I or anyone else on this forum can give you is to go listen to different subs.  Everyone hears sound differently and has a different perception of what is good and what is bad.  I, for instance, find the Infinity Kappa Perfect subs to sound a lot better than many other subs in the same price range and higher.  Someone else might think that the Perfects sound like crap though.  It's all in the ear of the beholder .  So my advice is to go listen to what you can and pick what YOU want.

Well that's the best I can do for now.  If I said anything wrong, misleading, or unclear, then please correct me or ask.