Excellent post Brad. As explained above, there is too many factors to pin point a set frequency range across the board for everyone. I have had mid-bass drivers in prior installs that had to be cut off (slope varied) as high as 70~80hz. Where as another more recent install, where I had a larger rear cavity for the drivers, lower frequency capabilities, and substantially more deadening/dampening, I was able to run down to 40~45hz with ease. If you can, and your drivers are able to, aim to get them to cross over as low as they can. This will help bring the lower frequencies forward as part of the front stage (if in system with no rear "stage"), and lessen the range of frequencies the sub woofer has to cover. As always, it takes testing and adjustments, because you don't want a harsh separation between where the mid-bass's roll off and where the sub woofer comes in (meaning dialing out any sharp peaks in output when the sub woofer come in).