as u can tell by my name im gonna say alpine. The type R(old version) is a great sub for SQ and SPL for the money. You can buy 1 12" for 100 dollars on ebay with the factory warranty sealed in the box.
infiniti and image dynamics should be on that list as well.. alot cheaper than what the jl, re, or eclipse will run you.. but then again it all depends on the model..
based off your first post i take it your mentioning the names becuase your reading posts and not becuase you actually have first hand knowledge of the companies.. i would stick to the infinti or highly reconmend the image dynamics.. great SQ sub. If you have the room for 1 sub.. make it a 12"
I want a jl w6v2 quality sub.... if another company makes a nice sub like that for cheaper... id definately look into it. ill look into those subs. which model? most companys have low end subs and super nice subs. im gonna go look around for them. Thanks
usually its so that you can wire it to certain amps...say your sub is 1000 watts peak and 500 rms....if you have an amp that is 2 ohm stable and it is 500 watts at 2 ohms...and 250 watts at 4 ohm....if the sub is 4 ohms it can only get the 250 watts...but if your sub is 2 ohms you can get that whole load...thats if its SVC...if its DVC...if you get a DVC 4 ohm sub....then you can wire it in parralleel and get 2 ohms or wire it in series and get an 8 ohm load