Depends on what you mean by noticeable?
Keep in mind the Brute Factor is not necessarily meant to go "FULL FORCE!!!!!" on everything. It is just a feedback circuit from what I understand, so lower levels will not necessarily be insane distorted sounds.
On mine I only notice very minimal "warming" effect up to about 30%. Then from 30% to 70% is where things start getting grungy, then 70% plus it pretty much goes all out saturated fuzz/distortion/bass.
I have noticed that the effects of Brute Factor vary quite a bit depending on the other things you have going on in your sound, so depending on resonance, cutoff, note range, amplitude... pretty much every setting on the MB, LOL... it will have a different "response" through the Brute Factor.
That's how it seems to me anyway.
So when you turn up to like 30-50% range you don't hear any kind of "warmer" / bassier boost in your sound?