I have no experience with the LSD/DXM combo, but i have done acid with ketamine and N2O a few times now. Ketamine and N2O are NMDA antagonists as well, like DXM. They both do not have opioid effects though, like DXM has, and no MAOI effects either.
The combination of NMDA antagonists and LSD is certainly a powerfull one.
I don't find it as deeply psychedelic as the combination of LSD and cannabis though. For me personally, cannabis is able to "unlock" something, to make you go deeper into the psychedelic mindstate, that dissociatives just don't have.
I think it has something to do with the nature of dissociatives. They do not just dissociate the mind from the body, they also dissociate the mind from parts of itself. It is like you are less present in the experience, even though the experience itself is quite powerfull.
The experiences i had with the combination of LSD and ketamine or N2O felt almost generic in a way. Void of meaning.
It is fun and all, bright lights and visual fireworks and stuff, but it just never seemed to realy go anywhere.
Just my personal opinion.