In case anyone is having the issue with rx500 series gpu's that can't reduce voltage below +/-0.9v, that is caused by the VRAM voltage. So if you have your vram at 2000mhz @ 1v, your core won't go any lower than 1v (under load, it drops a little). Because the default vram voltage on my sapphire rx580 4gb pulse is 950mv, the lowest possible voltage is 950mv. So, I did this:
this is not a problem of rx 5** series exclusively. rx4* have the same limitation: the core voltage is tied to VRM voltage (btw NOT memory voltage, which is always about 1v but only the mem controller as Wolf0 insists) also setting vrm offsets may help if available