A compressor before the octave pedal helps.
I also toyed with putting a passive tone control in front of it, so roll off the harmonics which might confuse it.
I've just got a Markbass Super Synth, whose octave function seems OK; it offers +1. -2, and -2 octaves and tracks pretty well. Not cheap if you just want an octave, though!
My other octave pedal is an Aguilar Octamizer and, as folks have mentioned, it's happy going to the C (or maybe B) on the A string; I also found that the sound breaks up if you hold a note and let it fade. I put a CP-1X compressor in front and that made a distinct improvement on stability and I seemed to be able to do an A too. The reason I chose the Aguilar over others I tried (MXR, Boss) was that it kept the character of the notes played, rather than producing a dull mush; however, needing to compress and/or turn down the tone kindof defeats that.
I think that doing octave below on the lower registers on a bass may be hitting the laws of physics.
Some years ago, I made an octave up pedal (I think based on something in Elektor magazine) and it was interesting; essentially half-wave rectifying the signal doubles the frequency and adds a distortion.
Another thing I've considered is trying a sub generator rack unit - I don't recall the brand though (dbx?).
And finally, as an adjunct, I've got a Minitaur, which can do some interesting stuff; however, I'm not very good at standing on one leg, so playing it is a bit of a struggle for me!