Late to the show but thought I'd throw out an option...
I have an Fx4, and by itself it's quiet. Really quiet. (Your Fx6 should be too). But I have a completely open back to my cabinet so even the slightest noise / vibration can be heard.
I use a cut sheet of NBR (Nitrile rubber) / PVC insulation. Typically this is used to wrap commercial ducts, tanks, pipes, etc. Think - the copper pipe insulation you get at the big box store, not the foam but rubber type, only in a sheet.
I use 1 (one) inch thick, as it will compress a lot especially under a water filled Fx4/6. Compresses about 75% under the pressure points (feet) of the canister. You can find it via a search, 36x48x1 for maybe 40 bucks.
My filter is now is completely silent. To the point of me having to look at the outlets / surface agitation to make sure it's even running. Does wonders for an air pump or anything else on it and does NOT absorb ANY water.