Earl Geddes investigated both symmetric and asymmetric placement of multiple subwoofers and concluded that asymmetric placement following a few basic guidelines was superior. Nothing against Todd Welti’s study of symmetrical placements, but there are other options.
As for whether or not to highpass filter the mains, the short answer is, it depends. If the main speakers and/or their amplifiers are being taxed, then it makes sense. In @audioquest4life’s situation, his main speakers have approximately sixteen times the cone area of a 6.5" midwoofer, and probably at least twice the excursion. Those interested can do the math.
Ideally any highpass filter in the main speakers’ signal path is not audible to the point of being detrimental. I have customers who have tried it both ways and use a protective highpass filter, and customers who have tried it both ways and do not.
When I design main speakers with subwoofers in mind, I tend to give them sufficient excursion-limited headroom that a protective highpass filter is optional instead of mandatory in most applications.
Duke