actually you will probably lose power (if you keep the same amp) because the ohm load on the amp will be 'lessened' i.e. you go from 2 ohms to 4 ohms, you lose 1/2 power (theoretically)... and if you go from 2 to 1 subs, you will lose 2-3db.

you are looking at a considerable loss. I doubled up my cone area and got a bit more power and a new box and gained about 4-5db.