notable computer hardware:

• 5900X
  
  
• 3090 FE
  
  
• 4x8 GB of Trident Z 3200 MT/s CL14
  
  
• X570 Aorus Master
  
  
• Corsair RM850x
  
  
• completely DIY 24-pin ATX and 12-pin Nvidia cables
  
  
• 250 GB 960 EVO, 1 TB 850 EVO, some 3 TB spinner
  
  
• Aquacomputer Farbwerk 360

(sort of) internal cooling hardware:

• Heatkiller IV Pro in black copper
  
  
• EK 3090 FE block
  
  
• Heatkiller D5 Tube 150 with vario D5
  
  
• two Heatkiller L-series 360 rads in stainless
  
  
• bunch of EK Torques
  
  
• ZMT!
  
  
• six Corsair ML120 Pro Whites as intakes, three Noiseblocker B12-PS eLoops as exhaust
  
  
• external bypass for system use without the MO-RA
  
  
• a temperature probe is taped to it

external cooling hardware:

• MO-RA3 420
  
  
• nine Silent Wings 3s
  
  
• Aquacomputer Quadro with plug-type temperature probe on the rad as control method
  
  
• D5 vario with Watercool top
  
  
• separate ATX power supply
  
  
• Koolance QD4s and a random assortment of Barrow and EK fittings
  
  
• ZMT!

I think that about covers it. After four years of using a Corsair 570X case, I could no longer stand it and had to replace it with something else. I wanted to use the Heatkiller rads and the O11D XL is both capable of housing them and sufficiently boring. I have a 360 GTS lying around, but given that I am going to use this system with the MO-RA, I can't be bothered to try and stuff the rad into the side. It's plain not needed and entirely more effort than I am willing to exert.

It appears that remounting the block and using the intended flow direction fixed my GPU-coolant delta of 15-16 K. It now seems to be in the order of 9-10 K.