No it's not like an attenuator. In respect to its ability to control level, it acts as another master volume. The main purpose of the C-lator is to create a buffered effects loop for the passive loop of the OTS amps. But, either due to purposeful design or not it has a really nice added effect (by itself) on the gain structure of the OTS amps and can be very useful in taking the amp further in it's tone nirvana. IMHO.
Complicating it a bit further is that some of the OTS amps that Nik designed benefit from it more than others. I think the FM is one of them.
Whether or not you would like what it does is hard to answer. When you say you don't use effects it makes a even harder to answer. If you ever use any effects in the loop that are not impedance matched very well it is going to degrade tone.
If you like to play with the amp over driven very much, I think it is a great addition. I love having it with the HRM.
I agree. I got the c-lator for my BM because so many guys on here said it brought another layer of goodness to the tone of the amp, and they were right! I never really used many effect pedals in the last 15 years or so, outside of a fuzz or OD, but since getting an amp with a loop i am starting to use a few now (space echo, reverb, analog chorus) and have found that i really enjoy using them. That being said, just the benefit of using it as a global master was enough for me. I could turn my masters up a bit and control the whole shootin match with the c-lator. it comes in VERY handy on the OD channel since i get a huge volume jump with the HRM being bypassed.
Gregg