I know that the general rule is to timbre match the front and center channel speakers. Obviously, the easiest way to do this is to match speaker brands and lines. However, if I want to mix and match speaker brands, how would I go about timbre matching? Do I need to do this qualitatively by listening to the speakers together to see how they blend? Can I instead look at certain measurable physical characteristics of the speakers to determine whether they are timbre-matched? I would love to be able to do the latter--it is a lot easier to search online for matching technical specifications than to lug about 15 different speakers home for tryouts.