I have recently purchased an CASA III with extreme DACs and Standard Dacs. My speaker in the front are 2.4 and SCS3 for center and powerplane inwall for rear. I am doing 5.1 system. I am thinking about adding the Thiel SS2 sub to the system. I was told by the dealers and Thiel that I should also get the Passive crossover for $500 more. I am confused because I thought that The CASA III preamp has superior x-over. Is this a marketing gimmick or do I need to buy another external crossover. Does anyone know about this/. Thiel and Theta users help?