Reports of First Plateau and Second Plateau doses from friends indicate that in their experience, using cannabis together with DXM completely eliminates any of the "gross" feeling if it's present, and increases the euphoric aspect of it.
I don't know of any reports from people combining cannabis with Third or Fourth Plateau doses, or with the Plateau Sigma technique.
I like the quote from the extensive DXM manual discussed above. About 1/3 of people love DXM, about 1/3 hate it, and about 1/3 don't really care about it one way or the other. Of the 1/3 who enjoy it, there are some who are mature and use it responsibly (for serious introspective purposes, or for the euphoria), but there are some who love it so much that they abuse it like an opiate. It can become rather psychologically habit forming if someone uses it every day or every other day. However, responsible use of it for the euphoric effects is possible. I know a friend who robo trips up to the second plateau every few weeks or so, just for the sheer fun of it, and loves every trip. However, she reports that she has no interest in using it more often.
So, the effects of DXM differ hugely from person to person.