+1 on the dod octoplus. It sounds lush. I find though i dont always want to go lower than the A string with an octave pedal because its kind of pointless. You tend to always loose the sub and all you hear is the note and no beef!
However, dudewheresmybass has the right idea with a subharmonic generator. Lots of beef!
Also, the Akai deep impact octave down sounds good too.