I can't see how a built-in sub can compete with a good separate sub. There are also various other arguments against built-in subs - the most important probably being that it is not possible to position the sub at the best place in the room for the bass because the built-in sub has to be, by definition, positioned at the place where the main speakers are. Given that those speakers have been placed for good imaging etc, it is most unlikely that that is also the best place for the subwoofer(s). Also, with a built-in sub, it is impossible to make the decision to upgrade the sub alone - one would have to replace the mains which could easily be entirely satisfactory in other ways. Even in a two-channel system, I would far prefer to use speakers which excelled down to 80Hz or so and then hand over the heavy bass lifting to one or more capable subs. There are no (main) speakers made that have the bass capability of my dual Submersives, nor the freedom of placement - and this also applies to many other competent subs too.
So yes, if I had the speakers in question, I would treat them as 'normal' speakers and cross them over at 60/80/100Hz (after testing which is sonically best) and let my subs handle the bass. In that sense, they would be good main speakers and the usual Audyssey setup considerations would apply.
You couldn't be more right: the built-in subs of the BP or Mythos ST/STS line are only 'competitive' with real subs if you have a small room where you can't have real subs, don't care about reaching down beyond the 30-40 Hz range, or strike it lucky with room placement that supports having the subs in the front of the room (which as you point out is unlikely). And you're allergic to the measurement rabbit hole . You have to start with these as 'full range' speakers and forget they're 'subs'.
Having said that, the real value of powered speakers IMO is the ability to control on-board volume via level matching for the powered section prior to Audyssey runs, which allows somewhat more power to the mid-bass between the crossover from bass management and the true mid-range than you might get from a non-powered speaker. But even so, there's a certain black box element to how the bass radiators in these speakers help you there with harmonics; I once found a review of GoldenEar towers (similar construction approach, designed by Sandy Gross that did the Mythos ST/STS line when he ran DefTech) that suggested there's a natural drop at 150 Hzish, that could only be compensated for by having more sub gain and SPL at the 80 Hzish region. DefTech is pretty fuzzy about how this exactly works, but my guess is that since Audyssey EQs to the -3db point of the speaker, there's a gentle slope from below the crossover to points about the crossover that help you on that front. So getting the right 'blend' of subs and speakers is indeed tricky.
Of course, IME even with three powered speakers (Mythos ST fronts and a CS-8080 HD center) and two HSU subs in my own case, you're somewhat overpaying to get this capability vs. speakers that might do better in the 80 to 200 Hz range. However, the overall quality of the BP/Mythos line is worth the money as a whole IMO. Just keep in mind that in an optimal world, you're not buying them to be subs.
All good points. Personally, I just can't understand why, these days, anyone bothers with speakers that purport to go below 80Hz (I am speaking generally not referencing the DefTechs). It is so much easier to design a speaker that only needs to work down to 80Hz or so than it is to design a "full range" speaker and the former is likely to be a lot less expensive too. Couple it with a competent sub and you can have a speaker system that easily delivers from 20Hz to 20KHz without problems. Using components designed and built for their particular duties just seems to make more sense to me.
I know that in the 'old days', when we using stereo pairs of speakers, things were thought of differently, but nowadays the idea of using speakers that handle 80Hz upwards and a sub for the frequencies below that is so well-known that it surprises me anyone bothers paying all that extra cash for so-called 'full range' speakers (which are usually anything but) and still they are not getting the very lowest frequencies, or certainly not with any worthwhile SPLs.
I guess if the system is for music only, one can argue that there isn't all that much content below 40Hz - but the fact remains that there are still two octaves below that and many movies these days dig that deep routinely. So for HT use this would seem to point indubitably to a sat/sub system. And if you have it for movies, then you may as well use it for music too. Just thinking out loud while we wait for someone with Audyssey-related issues to drop by.