Hi!
I am a professional bassplayer and I play bass for nearly 30 years now. The answer to your question is: Paul used a Rickenbacker 4001/s bass for "Taxman" and for example "Penny Lane". The sound came from using a bassguitar pick, and the Rickenbacker bass has small sponges (not kidding...!) glued on metal plates that you can "switch on" by using a screwdriver and let tthese sponges touch the bass strings from underneath.
I used this sound a lot in a beatles cover band, too. Without any question, the "Taxman"-sound is NO Hofner Violin Bass, its a Rickenbacker 4001/s. Paul used this bass with the beatles only in the studio (never live) and you can see this bass on the videos of "Hello Goodbye" and "I am the walrus".