I'm really glad I found this thread, becuase this has been something I've been thinking a lot about recently.
DXM is definitely one of my favorite "mind" (as opposed to pleasure) drugs, to a large degree because of its alien feel. it seems to diverge from the standard, more natural feeling psychoactives such as tryptamine/phenethylamine psychedelics, opiates, alcohol, etc. The best way I can describe it is as though DXM is the Z axis on a Cartesian graph, affecting me in ways that no other drug does.
I generally have a hard time remembering too much of my DXM experiences, which are typically low-med 3rd plateau (what I try to aim for, dosages anywhere from 350 to 700 mg). However, certain themes seem to keep cropping up in my experiences. First of all, the "dex-space" for me has a unique feel that is the same every time, as though I enter a universe similar to normal consciousness in that continuity of feelings or events exist, as in real life (sorry for being confusing, its very difficult to explain). Also, my experiences tend to bleed into one another, as I remember my last DXM experience with great clarity when on DXM, although with sufficient effort, I can revisit any of my prior experiences that I can remember, to some degree. The closest analogy is that of a 19th century theologian suddenly transported to the realm of the the Hindu or Grecian pantheon (I second the feeling of grandness, NMR). However, it also (often) has a distinctly dark, sinister, forbidden feel, as well as feeling very artificial in the body. Because of that, I labeled the feeling I have gotten while exploring the dex-space as "Subterranean-Hyperspace" for its feeling of being aware of the scope of the universe and at the same time in some indefinable way underground, or immensely small. My friend who almost always trips with me on this substance described it best as: "Putting your head into a little metal safe that barely comes up to your knees, and then toppling in because it contains the whole universe."
Two of my most interesting thoughts regarding DXM:
First, as to the OP, DXM does seem to put me on a level of "reality" much deeper or "more real" than many of my psychedelic experiences (the only exception being DMT, which I feel is almost the polar opposite of DXM). Although, unlike yours, it makes me feel as though all of our waking reality is in fact a giant manifestation of something else (often a computer program, or network of servers which generate conscious experiences) and in particular, death, and harsh realities seem not only unimportant, but difficult to define. DXM feels to me as though it shows me the basic programming from which the universe is extrapolated, which feels to me in some way like "forbidden knowledge". The theme of Cosmic programming and the structure of the universe recurs frequently.
Secondly, DXM has a particularly digital feel to it (not a pun on robo

). While I am in the head space, My only emotions are of wonder and explorations, and a deep sense of internal clarity; my body is weightless, and the "organic" emotions of fear and pain are nonexistent. I am utterly incapable of understanding humor in any form, and, although my logic is impaired by my perception, it still seems to have a continuity that can be followed, although not remembered. Also, the closed eye visuals remind me very much of high definition digital or computer generated images, crystal clear and at the same time slightly misty, particularly when combined with cannabis or morning glories, which enhances the DXM-themed CEVs TREMENDOUSLY (and often gives them more color, as they are typically black and white when using DXM alone). Often they are random images, such as Escher-esque planets and staircases, machinery, turning gears, solid objects melting, wooden paneling, metal and electricity. One of my most traumatizing and at the same time fascinating experiences was when I had a CEV of a thick triangular coil of cold wire on some sort of inverted dais made of metal, which I then "realized" was an extra-realistic device which was generating the totality of my consciousness (and yes, I realize that I sound insane

). However, this feeling has been a recurring theme in my experiences since then, and has made for many interesting insights, or at least perspectives, on humanity, the natural world, and so forth. Also time travel, becoming other individuals, finding oneself in different part of the earth/planets etc. are all effects and themes (albeit lesser ones) of DXM.
In closing, sorry for the length, I've been meaning to write this up for ages and have never gotten around to doing it properly (this is actually a summary). I need to start posting more in trip reports, I could go on for days...

Peace.