Sounds like me the other day. I couldn't get the lighting right. I ended up selecting a wall in the room which wasn't visible and made it emissive and that lit the scene perfectly!
I generally use the denoiser. I don't think I have played with the noise degrain filter much. I have run many, MANY tests with denoising, no denoising and I find the difference in quality to be barely noticeable, but the excess time needed with no denoising I find just not worth it. Lately I have done between 400 and 800 iterations, denoising on the final iteration only. I just experimented the other day, running tests with denoising, no denoising, and various pixel filter settings. I had one scene where I did have to double my normal 400 denoising to 800, but otherwise the difference wasn't worth turning it off.
Now I'll have to check out the noise degrain filter... and run a bazillion tests with it! LOL