You do not need dx driver for this
In my case i needed DX-Drivers for this, as "allow applications to take exclusive control of this device" does not fix the issue for single-client-Asio4All, which was the only working Asio-Driver on my system.
Gladly i found a solution in viewtopic.php?f=181&p=398897#p398897
to get back my old-loved Multi-Client-DirectX-Full-Duplex Asio driver by reinstalling Cubase 6.
(To all those folks saying "DX-drivers are *flower*, they have high latency, do not use them": When i'm just mixing i don't care about high latency, but i do care about multi-client-audio. And there are also reasons for not always having a separate interface with its own ASIOs plugged in.)