I think you are trying to joke now? I tell you how challenging DDR5-6400 is and you want to try DDR5-6600? While you haven't found anything truly stable yet?
Like i said before, the following factors all affect if the RAM can run at a certain speed:
- Your mainboard (PCB layer count, PCB trace optimization, RAM slot layout, component selection, RAM VRM etc.)
-> Good, you have a nice board
- The mainboard's BIOS optimizations
-> So-so, previous platforms had less RAM problems, but this is the first platform with DDR4+5 support
- Your CPU's integrated memory controller (IMC), quality depends on the individual CPU (silicon lottery, not only for CPU core quality, also for Uncore/IMC quality)
-> Unknown, but probably not top notch
- And finally, the properties of the RAM modules.
-> They are binned for DDR5-6400, but all the other factors need to be good too.
DDR5-6600 is going towards the extreme DDR5 OC territory, i don't know what makes you believe that you can achieve that so easily. 1.5V for the DRAM voltages is extremely high, it would even be high for DDR4 already, but for DDR5 it's really a lot. You start to need active RAM cooling around there, otherwise the RAM might cause errors once it heats up. For the other IMC-related voltages, you might degrade the IMC inside the CPU if you run it daily with excessive voltage.
For three pages we now have tried to doctor around with your RAM. Maybe it's time you post a full, detailed list of all your hardware, exact models of everything, and maybe you need to even consider buying a different kit of RAM. But you might end up finding out that the RAM is not the limiting factor, and it's one of the other things i listed.
DDR5 is in its infancy. You cannot expect the level of maturity that you know from DDR4, especially not above DDR5-6000. So if you want the best chance of stability, stay at DDR5-6000 and below, and then it should not be too hard to make it stable. That should need no more than 1.35V for the DRAM voltages, and the other IMC-related ones, you will have to test.