So I managed it somehow... Though what it has lost in elegance with the BQ fans, it kind of looks a bit more badass now.Warning: Spoiler! (Click to show)
Had to cut the a piece of the fan clip's end, so it does not hit the blade.
Now a single XF140 at lowest possible RPM gave me 57C at the warmest core at a 20C room with dual intake, dual top exhaust and single rear exhaust be quiet! fans at 500 rpm. Without the top exhausts that I used to clip to the R1 ultimate - the XF140 was scoring 59.
Now the dual 120mm Silent Wings2 at 800 RPM deliver 64C at the warmest core at 21C ambient. Now put 1-2 degrees due to my sloppy paste application this time and maybe the tiny dust film and there you have it. Dual 120 mm Silent Wings delivering 3 C less performance at the fraction of the noise of the XF140. And the heatsink does not vibrate anymore.
The noise is now so significantly lower, that I can hear a faint resonance. I can't identify from where it comes exactly. I will wait till 2-3 AM when things are all quiet and try and check each component and each fan.
Just for extra information - I had a check of the ambient noise in the room - 19 db (which is the lowest threshold of the device i was given) in the night. So I guess it is even lower.
But I guess I will be searching for a singe 140mm fan replacement for the XF. The dual 120mm Silent Wings2 do not appear to be that effective, since they don't cool off the two most outer heat pipes very well.Edited by Shneiky - 10/18/15 at 11:19am