Sloped baffle

@Bifwynne

For the first part of your question, you misunderstand. Pass band is the part of the frequency the driver is covering, unattenuated, within the filter. Actually, I used the term technically incorrectly in the BSC context since that is attenuated long before the crossover point. Driver rolloff caused by inductance usually occurs out of the pass band but is still important. If a driver could, realistically cover from 35 to 20 KHz, than it would require very little inductance. There are drivers with little inductance, relatively, like the Satori MP16, but the numbers you mention are bordering on some AVR brochures :O

The second part is beyond me, even if I could understand the question.

I have a question. If an 18kHz sound left its' source and a 30Hz sound wave left its source at the same time. would they both get to the listener at the same time?

How fast is the train travelling?

"If an 18kHz sound left its' source and a 30Hz sound wave left its source at the same time. would they both get to the listener at the same time?"
Yep, its not the speed of frequency, its the speed of sound. All frequencies travel at around 1000 ft per second... I'd have to look it up to be exact, but it also varies by sea level. The difference is how many times a wave will hit you.

Let's assume we have a single dynamic cone speaker with a pass band of 35Hz to 20K Hz. Let's forget about high frequency beaming and cone breakup. Just assume this hypothetical speaker has a flat frequency response within its pass band, as measure on axis.

Bifwynne, I agree with Ngjockey here that if your hypothetical speaker has a flat freq response between 35Hz & 20KHz then all signals in this frequency region will pass thru minimally unaltered. That's the meaning of "pass band" - frequency passes thru minmimally altered. This, of course, means that in the 35Hz-20KHz the effect of the speaker coil moving inside the magnetic field poses no issues. So, there should be almost zero phase shift in the 35Hz-20KHz region.

Is there a frequency range where a speaker is phase coherent

yes, its phase coherent inside its pass-band. In the case of your hypothetical speaker it's phase coherent within 35Hz - 20KHz.

or does phase nonlinearity increase as a function of frequency ... period??

yes, it does. And, in the case of your hypothetical speaker, phase coherency degrades below 35Hz & above 20KHz both of which are outside the pass-band of the speaker/driver.

If the answers to all of these questions are -- yes, then it seems to me using 1st order X-overs and sloped baffles is at best a rough justice engineering response to a problem that is inherent with dynamic speakers that use voice coils.

Bifwynne, I'm not sure that you realize what the benefit is of using 1st-order x-over? The benefit of 1st-order x-over is that the PHASE DIFFERENCE (not talking about the absolute phase of a certain frequency) among all the signals in the audio band (20Hz-20KHz) is constant.
So, you have a music signal coming into the speaker. This music signal is a complex mixture of many frequencies. All these frequencies have some absolute phase that is different from each other. Further, each frequency has some non-zero phase difference with another frequency in this complex music signal. So, this whole complex music signal now goes into a time-coherent speaker as an electrical signal & comes out as a sound pressure wave. The phase difference amongst all the frequencies in this complex music signal do not change (i.e. remain the same) if the speaker used a 1st-order x-over. This means that the timbre & harmonic structure of the music remained unchanged as it passed thru the speaker. No other higher order x-over can achieve this i.e. higher order x-overs change the phase diference among the many frequencies of the music signal as it (music signal) passes thru these higher order x-overs.
So, ifffffff, the solution is a moving target (as you wrote) a time-coherent, first-order x-over speaker is the least damaging (IOW, the best compromise solution to a moving target problem).
hope that this helps some.....