When undervolt with claymore 6 rx and 900 900 consume 860W ... to trick and make undervolt in phoenix as above says, you start claymore, overdriventool, close claymore, start phoenix.

But phoenix will consume 875W  ... so 15 more. Yet, is more stable for me (27H and no crash), while lately claymore crashed rig when using low voltage... after an hour, 17 or more.

Conclusion : in my case, undervolt works better with phoenix. Waiting for 2.7