Can't change the preview volume in media bay with oberhausen_bx

Hello dear forum,
I work with Cubase Pro9 and recently installed the vst oberhausen_bx. Now I noticed that with this plugin in the media bay I can’t change the preview volume with the slider. With my other VST’s (UHE, Omnisphere and hardware synths) this works perfectly.
Does anyone have an idea or workaround?
Thank you for your support.
Lars