"I'm saying I think probably it is worthwhile to have closer-spaced overlapping midwoofers for modal smoothing, and further spaced subs." i.e. non-Welti, or Geddes, a sort of Parham arrangement.

Below 100 Hz "integration with the mains" is irrelavent - it is insignificant. Thus, to me, your "issues" are simply not relavent. And your argument for symmetric placement is weak because it will always yield a higher spatial and frequency variance than a random or partially random placement will and there are no "integration" issues.