I don’t know that I would say the signal is degraded by going through a tube amp or preamp. Every piece of equipment affects the sound in different ways. It all depends on what kind of sound you are looking for. Of course, mating a low cost Adcom amp with something like a McIntosh CD player definitely has the possibility of "degrading" the sound. However, it also depends on the person’s budget.
It sounds like you are wanting to try the MC275 tube amp. Tubes have their own sonic signature and can be different than solid state. That being said, the McIntosh sonic "character" is more warm and laid back (can also be described as "rich" sometimes). It is not my preference for sound character as I want more neutral and high-resolution sound, but many love the McIntosh sound.
You should go out and see if you can find someplace that has the MC275 to listen to. It might give you a better idea of what to expect. Then, if you like what you hear, buy an MC275 and connect it directly to your MCD550 (using the MCD550 volume control). It might be that you are fine with this combination. If there’s something missing, then post a thread in the preamp section describing your system and then describe the sound that you are after.