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.
Tube amps lend themselves more naturally to distortion than solid-state. When overdriven, tubes produce more of the nice-sounding even harmonics, while transistors produce more of the odd harmonics, which don't sound as good. Early solid-state amps distorted very un-musically. Bass is generally played clean and requires a good deal more power, which would require a very large, heavy, expensive, and delicate tube amp, so solid-state became a good alternative for bassists.