In polarity with a DC signal with the speaker occupying the same space,
the 6 dB differential is absolutely correct. With a complex phase
signal, which many commerically available loudspeakers being
non-coherent at best and which differ slightly between even sequential
serial numbers, the issue thus becomes significantly more complex.

This can be demonstrated with two stacked identical loudspeakers fed
identical, but with a relative polarity reversed, signal. This was one
of Don Davis's favorite tricks in his Syn-Aud-Con training sessions.

One of the easiest ways a non-technical person can evaluate a speaker
for obvious phase anomalies is to playback a full bandwidth swept
sinewave at a relatively low level. These are available on dozens of
commercial test CDs. If you hear "birdies", you have a phase problem
at that particular reproduction frequency. A birdie is immediately
obvious, sounding something akin to a Looney Tunes cartoon sound
effect. BEE-OOO-WOOP.

To the best of my experience (I've tested many), only some of the
planar electrostatic and plasma speakers have truly decent full
bandwidth phase responses over the majority of their bandwidth while
not exhibiting compensatory frequency anomalies. A few (a handful)
dynamic speakers from truly capable designers are also on the market.

This is where digital filtering (once it is fully understood) will
eventually take over the commercial signal processing market- the
theory being eventually we will be able to alter frequency without
requisite phase anomalies as happens in the analog world.

To listen to a high quality reproduction system with excellent phase
response is a truly great experience. It does transport you to a
different world, not unlike viewing a real painting masterpiece. A
picture in the book can only bring back memories of the original at
best.

That being said, the major thrust in the commercial market today is not
reproduction (including phase) accuracy- note how many commentators
marvel at higher bandwidth iPod iterations while exclaiming the virtues
of its near perfect performance. One wonders just how many Stax
headphones are actually connected to iPods in this world.

I'm sure we'll have to wait this trend out. Ten years ago 44.1 kHz
sampling on a CD was not good enough and we eventually ended up with
competing DVD Audio and SACD audio products. How an iPod sampling
(generally sourced a 44.1 CD and then heavily compressed) at 128kbps or
even 320k reaches perfection pushes the world of the illogic back to
flogiston theory. Mind you the DVD Audio bit rate is 9.6 MEGAbits per
second.

One of these days I hope I'll read a review of an iPod's technical
performance using high end test equipment. It would be great to see
bandwidth, polarity, phase, THD+Noise, S/N ratio, etc.

Yet I digress...