My Niagara 1200 performed the opposite of what's been described. Quieter background, no restrictions in dynamics, lower noise floor, a very slight film/hash removed that I didn't notice until it was gone, resulting in a smoother, natural flowing sound.
I believe it's completely passive as there's no transformers and provides surge protection as well. If anything, it sounds like it past on the troubles you had up the line that were beyond it's passive design.
All the best,
Nonoise