As Jim already noted, localization is highly dependent on the subwoofer itself and the room interaction. Most people cannot localize a subwoofer below 120Hz if the sub is capable enough to pressurize it. Again, it all depends on the room and the capabilities of the sub(s).
Concerning separates, go for it; I don't think you'll regret it. If you like Frank's stuff you don't "need" a big hog type amp; just get something smaller like the Insight+ 240 or 260.
Regarding the lower bass frequencies, this is where a subwoofer comes into play, as you can do a couple things differently.
1) You can place the subwoofer where it sounds and measures the best. It is unlikely that the best place in your room for the speakers (imaging, a wide and deep soundstage, the least reflections, etc) is also the best place for achieving a good bass response. Crossing to a sub (or just adding one) can provide the best of both worlds. The one caveat is it's no easy task to achieve a great blend, so you'll have to work at it.
2) With a subwoofer, you can apply Parametric EQ and create a bass curve tailored to your tastes. Like Jim mentioned, we don't hear the lower bass as well as the upper, so you could apply an arced curve with an EQ, with the lowest bass being a little louder than the upper bass. Although it may not measure "flat" it may sound flat to your ears, which is all that matters.
With all that said, the subwoofer really does need to be good quality. I love Dr. Geddes work, but the one thing I completely disagree on is his claim that the quality of the subwoofer doesn't matter; this is absolutely untrue in my experience (and many others). Otherwise his theories and implementation are spot on. So, knowing that, if you do want to get a quality sub you're in the right place: Jim is also a master subwoofer builder.
