Fidelia shouldn't add appreciable hiss to your audio chain. If you have 'Fidelia Advanced', what are your dither settings? Also, what is your gain staging like? I'd contact support if you can't quickly get to the bottom of it--they're very responsive.
Being that I helped design FHX, I think it's awesome!
Lately, I've found it hard to listen to music on headphones without it; FHX makes the experience much more natural IMO. It also adds monitoring controls (mono, phase, swap L/R, solo L/R) that I could only perform on my ~$3000 Crane Song Avocet before, so to me it's definitely worth it.
My best advice would be to demo FHX and see if it suits your listening situation and your music.
Best of luck,
- Devin