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Although all of it has no relation to digital crossovers but still:

cv wrote:

On a vaguely related note: I dunno if you realise this, but conventional (ie excluding horn or large area ESL) speaker systems, even if measuring flat amplitude, introduce 90 degrees of phase shift; at 100Hz, this is about 3 ft worth of delay, and it gets worse the lower the frequency.

Sure they do, all of them and there are many reasons why and how. However, the fear of “phase shift” should be properly understood. A system should be phase-coherent initially, design-wise…. but THEN a system MIGHT introduces some beneficial phase anomalies. However they should be intentional, well understood and evaluated. By injecting into room some “strategic” LF phase irregularity it is possible to get some very “interesting” results that might very effectively overwrite/mask-out some other probolems of sound reproductive environment.

For an experiment walk around a not-tightly seated large symphonic orchestra while the’re playing and see how your inner-you reacts to the LF delays.  For instance if the orchestra positioned with bass strings and with tubas at upper right (the typical contemporary orchestra positioning) and if you’re 20 feet in front of the orchestra on the left then you have a approximately 50 feet between the first violins and bass section. If disregard the gross luck of tonal balance in this case you might start to walk back and forth, right and left and you might find out then that it is not about a “correctness” of the delays but rather about the audible benefits. Even considering the phase randomanization in the concert hall you still might see that the delays might be a quite powerful EXPRESSIVE TOOL. Why do you think that managing delays within our playbacks we are in very different position then conductors who manage the delays of the orchestral groups? Good conductors recognize the idiosyncratic acoustic conditions of the specific performance halls and tune thier orchestras accordingly. We should do the very same with our rooms and with our dedicated to the specific frequencies channels.

cv wrote:

From this standpoint, I never understood why users of multi-tower systems (eg the big Alons, IRS V style etc) always seem to have the woofer towers positioned behind the mains. Equally, "time-alignment" of the LF with the mains is a misnomer.

Come on Chris! Those systems are a celebration of consumer deception over a common scene. Those multi-tower systems system never bult to accomplish any sound Result but rather to make a product presentable, large, size-tangible and be able to convert the presumptuousness of a products into money. Theoretically it should be wonderful: a company manufactures a LF-restricted loudspeaker and they provide as specially designed LF module that should be used with it. Sounds all kosher, does isn’t it? In the reality: none of those “specially designed LF modules” sound even remotely interesting or even marginally acceptable. They all made to bypass nothing further then a primitive reference point of industry’s quality control - the audio reviewers layer, and there were no further more noble objectives in the heads of their designers. Listen to at all those Martin Logan Statements, Pipedreams, Genesis, Alón Exotica Grands, Gryphons and the rest of them with passive dedicated LF sections. Why they all sound do predictably horrible? Evan if the companies go for looked-like-no-compromise pretensions then they still allow own stupidity and own greed to overwrite OUR listening benefits. Look for instance at the David Wilson XS woofers. With 800 pounds of a wonderful built inside of 45cu-feet of those sarcophagus and the price tag if I remember correctly $29000 each… How come that they still introduce a half-ass solution? Why in the name of God, using a pair of amassing Aurasound 1808 drivers per channel, they went of ported enclosures? David told me: “We have +20dB output ay 20Hz compare to a sealed enclosure”. So, what? Does he think that anybody drive them with 2A3 SET amplifiers? Or a person who have spent money for them and was able to accommodate a pair of them in his listening room would ever care? A pair of 1808 have 101dB sensitivity and in this enclosure they might be an spectacular LF section but… the “industry” again converted any would-be-good-intention into a “popcorn blowing machine”….

cv wrote:

Least compromised approach is proper horn loading to get the delay vs frequency (which is what phase is really) flat and the get the mastertapes and play them back on a machine with 2 heads to get your delay :-)

I actually considered the approach: to try a very good tape delay unit from 70s. If I would need to delay one channel I would probably go there (I do like how tape sounds) but I would be hesitant to do it with full range.

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