Thats a hifi amp not a top hifi amp (dsp carver). Five weeks ago I ended up buying a new one for £340 after searching for 2 months for a used one and now this one turms up :x
There are loads of them on ebay:
http://www.ebay.de/sch/i.html?_trksid=p2050601.m570.l1313.TR12.TRC2.A0.H0.Xbmw+z4+verstarker&_nkw=bmw+z4+verstarker&_sacat=0&_from=R40