It took me a few days (and a few rounds of jumping between driver/CNext versions) to realize WattMan disables the GPU clock/voltage controls whenever your software is running but enables them when it's not.
I wish to try and undervolt my 480s...
Are you sure that "-tt 0" does not work? Moreover, ADL for 480 works only in v6.0.
I am using v6.0 now but I had the same problem with 5.3 (didn't try older ones with these cards).
Yes, I tried to add "-tt 0" but it doesn't seem to help (I only tried that with v6.0 though, not with v5.3). The GPU clock/voltage panel of WattMan gets grayed out whenever I start mining and it always comes back yellow (operable) when I stop mining.
I am not entirely certain it's related specifically to the hardware monitoring/controlling capabilities of your software or WattMan locks those controls under any kind of heavy computing loads. But it seemed to be a good "educated guess" at first if WattMan tries to "hand over the controls" when something else polls the hardware (think about third-party software like AfterBurner and GPU Tweak II, etc).