audio amplifiers: +/- X VDC vs 0-X VDC

I've noticed some audio amplifiers, like this one, need +/- 35VDC:
http://www.national.com/mpf/LM/LM3886.html

and some amplifiers are happy with 9 to 18VDC, with respect to ground:
http://focus.ti.com/lit/ds/symlink/tpa1517.pdf

Besides the obvious difference in power, how come one needs +/- power,
and the other is happy with 0-X power?

In the second case, for "no audio signal", is the speaker suspended
halfway between rest and max, resulting in only half of the available
speaker movement range?

Michael

.

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