Luxman "u" is the previous generation,in the 8, 6, and 5 versions.
"x" is the current generation. Currently only the 10 and the 3. Surprised to find the "u"'s still listed on the Luxman website. They will be discontinued. I gather that the intention is eventually to add a third "x" in the middle--5, 6, or 7, don't know.