Warning: this may not be exactly safe.
I once took 420mg of DXM (beginning of 3rd plat, I'm a small guy) and smoked a lot of H on the comeup. It was a total waste, once the DXM kicked in fully, I couldn't feel the opiate high anymore at all, even though I must have smoked at least half a gram of decent material. I just tripped like normal, but ymmv of course.
But I agree with the posters above, 60mg DXM and 50mg DPH 30-45 minutes before consuming your opiate of choice are my favourite potentiatiors as well.