I'm also currently using the 4 ohm tap of my MC1.2K's to drive my Summit X.

When I had my MC275 driving the Summit X, I found that the sound really opened up using the 8ohm taps, giving me greater air and clarity especially with the highs (mind you I only had a single MC275).

Tim - I just want to mention that when I moved from the 601 to the 1.2K...it absolutely transformed the sound of my Summits. The 601s are amazing, but...there is a transparency and effortless immediacy to the sound with the 1.2K that just took my system to another level. I'm thinking if you can get your hands on a pair of 1.2K to try out with your CLX...you may be amazed too by the results :)