First of all, I don't have a producer's ears. So when I say that there's absolutely no noise when using these preamps, take it with a grain of salt.
I have a specific problem that I tried to solve with the FetHead, and I solved it easily. The problem is that I'm getting some digital noise in my Focusrite Scarlett whenever I turn gain passed 1 o'clock. I don't know if it's a psu, or the nearby laptop or something else, but I couldn't get rid of it.
Now, with the FetHeads, I pre-amplify my mic, and I need just a slight gain from Scarlett, so the noise is gone! The sound is exactly the same as without the FetHead, just louder. I hear no distortion or noise. I'm using Lewitt small diaphragm mics that are especially noiseless, but I hope the others would behave the same.
I'd recommend FetHead as a cheap solution to any gain related problems you might have.