Cooling an amp in a way that's effective, quiet AND reliable is a lot harder than it appears. The fans that are featured as "super quiet" usually do not have the airflow , especially under any static pressure losses that a cooling system presents.
The D-800 has a fairly expensive ball bearing fan that's reasonably quiet (not as quiet as the sleeve version by a dB or two) but is very long life (about 50k hours compared with about 5-10k hours of a typical sleeve bearing fan). It also runs continuously at a slow speed because the resulting air flow allows multiple components to stabilize thermally together as a system. It's extremely unlikely to ever run any faster than the slowest speed, even at 2 ohms.
Back about 15 years ago, when I was working at another company, I led a research project (with the cooperation and support of ICEPower R&D) that specifically addressed a whole slew of thermal and dynamics/duty cycle management design approaches, which resulted in a US Patent relating to this application. Some of what we learned, and how they relate to bass guitar ended up being incorporated into the newer ICEPower modules.