At some point, I had the CTH-8550, and also wanted to move to the separates. I bought them both, but I took delivery of the preamp first, so I had the chance to try the CTH as an amp only. I can tell you, for sure, the CTH is a fantastic amp on its own
Once I got the 108, I noticed more refinement overall. Even though the 108 is not as powerful (on paper) as the CTH, I didn't notice any diminished dynamics, at least on the speakers I used back them (Dynaudio, Focal, Sonus faber, Magico).
Given that there's an mk2 of the pre, you might want to get a 108 right now, and keep the CTH as a preamp, until you can get the mk2 preamp, or score an mk1 on the cheap
Don't forget to get a Zeel cable, BTW. They're absolutely necessary to get the full benefit of the 108+18NS setup.
cheers,
alex