I wouldn't go so far as to say I can sight-read "anything playable." (I wouldn't be able to sight-read Jaco's version of Donna Lee, for example. ) But I can sight-read just about anything that I ever reasonably expect to encounter on a gig.
I'll bet a lot of us are in that same boat -- which probably says more about the expectations of arrangers, composers, and bandmates vis a vis bass parts than it does about bass players' sight reading ability in general.
I spent most of the 1990's playing in Debris (http://debris.rednotebook.org), an avant-garde instrumental ensemble that undertook seriously challenging compositions routinely (if not exclusively). We were reading the proverbial fly-poop. Music influenced by Elliott Carter, Anthony Braxton, Frank Zappa, Milton Babbitt, Ornette Coleman...dense, thorny, rhythmically asymmetrical and melodically angular, just nasty nasty stuff. By which I mean beautiful, of course. I have never in my life worked so hard to nail a bass part as I did on some of those pieces
...and yet 99.9% of the music I have had to read on any other gig prior or since has required maybe 25% of the sight-reading chops that band did. When you get on a bandstand and find yourself staring at a part that makes you think you're a B or a C reader, consider yourself lucky! It's so rare for bass parts to demand more than Somewhat Better Than Average reading that those charts should be cherished as an opportunity.
I miss the discipline of having to read those Debris charts. I'm sure my reading has gone downhill in the decade since, as I never have the need to work on the really hard stuff anymore.

