I have an Aguilar Octamizer and it is the only one I've heard which preserves the essential character of the bass being played. However, for me it only works down to C on the A string (others have said theirs goes lower) and it breaks up as a held note decays. That means you have to think about precisely how you are going to use it, but it can be wonderful when it's right; you'd have to construct your song around its capabilities.
I've also got an EBS Octabass (Mk 1); it's more stable but less characterful.
On the octave-up angle, I've got an EH Mosaic, which is really designed to make a 6-string guitar do a 12-string, but it does interesting things on a bass, albeit slightly synthetic sounding.
I've also got a Mooer Tender Octaver, which is a bit plinky, but might work in a mix.