Solo free improvisation for bass guitar in any real sense beyond random wankery is almost non-existent, and it's something that I've been checking out recently. "Free improvisation" is definitely not the same as free jazz, and it doesn't me playing "out" versus "in." It simply means creating a logical and well-constructed piece of music with little to no given structure. For examples of great solo instrumental free improvisation, check out Keith Jarrett's the Koln concert, or Dave Liebman's The Tree. Skip to about 4 minutes in if you want to check out the free improv without the chatter.