I've used a Pitchblack for my 6 string basses with never a problem. It's my 2nd tuner in fact. It's also 'true bypass', which is something you don't mention.
The Polytune is pants to be honest, not very accurate at all, and rubbish with a low B. I think it's supposed to be true bypass, but I didn't like it.
The best, is the Turbotuner, but it's not readily available over here. It's true bypass too.
Great reviews by the way.