A conversion neck is one that can convert an instrument from one scale length to another with no other modification. A common conversion is 34" to 32". By overhanging the 20th fret, or leaving it off, you can use the stock bridge location on the body, just adjust for intonation. Such a neck will work on any standard bass body. A conversion from 34" to 30" requires overhanging two frets, or losing one or both.
Active tone controls offer one thing that no passive control can do - boost. You can get them in one, two, or three bands. The latter can come with mid-frequency controls, which gives you nearly as much tone-shaping as your amplifier. Who doesn't want more of everything?