Yeah, the CT-125 seems a little more mellow than the stock tweeter, but it's been a long time since I swapped them out. When using a Marantz PM-5004 the tweeter was almost too relaxed. I recently switched amps to the IOTAvx SA3/PA3 stack. It's a flatter sounding amp and the high frequencies are more resolved, forward, and holographic than the Marantz.
Most people using the CT-125 are using it because it can handle operating down to 4500hz, but what I don't know is whether it's "good enough" or if the CT-120 is a clear improvement.