I use the Placater, L6 2204 Mod and ANGL Meteor amps most. For what I do live, I like the amp gain on most of these to be at around 7-8.
My Helix Rack is connected with a single 1/4" out to my Legacy 3 Return - Eliminated 4CM because with the noise using multiple amps, never went away. Switching to this mode has improved the noise floor dramatically for me but using any of the four effects loops on Helix brings all of the white noise and hiss back. I could technically use a Voodoo Lab GCX to add the delays and reverbs that I'm using after the Helix 1/4" out and before the amp return but then I lose the ability to have my loops modular and levels controlled with snapshots. Kind of in a bind because while I can increase the noise gates and squash the "kshhh" noise, but I need sustain for my playing.
I found that messing with the levels and mix in a SEND/RETURN block can take down the noise floor while still allowing the effect to come through. Not as strong, but enough for my Digitech FreqOut and Horizon Devices Precision Drive to still be effective. I've also experimented with three different types of isolated power supplies, sets of brand new cables, tried the Ebtech HE-2 in between multiple places and even run a higher end power regulator for everything (Furman M-8XAR). I made a video outlining some of my issues but since then I have even gone as far to take everything out of the rack and keep everything as separated as possible...eliminating proximity noise that I thought might have caused my issues. It has to be the conversion rates in Helix...HAS to be. Hopefully this helps...if anyone can help me, I'm all for it too! I'm close to the noise floor that I perceive as "acceptable" but am all for improving that if possible.