fafa otep: its either the voice coil, or some external grounding connection ek-ek. nangyari na rin sa akin yan. nakita lang na maluwag yung connection sa amp.

your other queries:

a) yes, subs are repairable.
b) 12"? tingin ko Targa pa rin....
c) hmmmm. baka mag-iba sound quality.
d) di ko alam yan....