Only trick I can suggest is creating a chain where a Low Pass Filter can harshly filter out higher notes, but feed the octaver to shift down only the lower end it receives. Its something I’ve done a few times in a DAW with stock Reaper pitch shift and not sure if it will be as clean, nor whether OC-5 does anything like that (I expect its more complex) but its pretty effective when I have done it! I believe its more a trick off bass boosters on old hi-fis without muddying up things at least
