the only bass octave i've really liked so far has been the aguilar octamizer, which is a modified ampeg sub-blaster. the aguilar has an added clean and octave filter (tone) control. i requested the ampeg awhile ago and there's a layout, but haven't had a chance to build it, so you might want to have a go at it. it seems more musical to me, like you're actually playing that note rather then it being processed.
side note, i bought an octamizer used and had to send it in for a repair. once i get it back i plan on opening it to see what they did differently to tweak the ampeg.