80Hz sounds like a good choice. If your surrounds are -3dB at 80Hz, you might even give 100Hz a listen on some good active-surround demo material just to see what happens. The result of moving the crossover farther from the lower limit of the speakers will be that the mains and center will not get any material below the crossover point as you point out, but the material won't be truly be "lost" -- it will just be passed to the sub, which in many cases is better capable of handling those frequencies anyway. It's all a matter of finding the point where the speakers and the sub blend together the best. Good luck!

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