After reading the great info that's been posted in response to my question, I'm eagerly waiting for the tubes that Mikhail is sending. The following descriptors, that are included in some of the posts, are especially helpful in my effort to explain the problem: "clarity and detail, while retaining warmth"; "full bodied sound"; "more airty"; "better image"; "better separation of instruments"; "enlarged sound stage"; fill-in the low end"; "apparent increase in clalrity"; and "darker sound".
So far, in comparing the R10 system to the HE90 system, I'm satisfied that both systems provide a full bodied and warm sound, with a large sound stage, Their main difference is in clarity, detail, and airyness. The HE90 greatly surpasses the R10 with regard to clarity, detail, and airyness, and these are the criteria that I want to improve with regard to the SDS/R10 system.
One more observation. After listening to most of my CDs, I separated about 50 CDs for sale on ebay, because they sounded unsatisfactory on the R10/SDS system. But, after listening to them on the HE90 system, it turned out that 45 of them sounded just fine, and are therefore keepers. Thus, unless I can improve the sound of the R10 system, I think that it will never bring me the enjoyment that the HE90 system provides.