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I wonder why this is not more common.

It should take a (cheap) microphone with a known response curve and a sine wave generator to calibrate any number of speakers in a room.

Most midrange A/V receivers come with mics these days to do this sort of calibration. For example, Denon's version of this is called Audyssey http://manuals.denon.com/avrx4100w/NA/EN/GFNFSYnuokgukf.php

It's quite common, actually. Yamaha, Onkyo, Denon, Marantz all have versions of it on nearly all their home-theater receivers.