I strongly believe now that the reversal in sub polarities I did during the Janus 1.0 system tuning was correcting a number of earlier mistakes, but the polarity of the subs is now non-inverted, which is what I intended them to be.
I most recently replaced the plate amplifier in the right sub. That was a couple years ago, and I decided I'd do it right this time for sure. So before hooking up the plate amplifier, I connected an extension wire to the sub driver so I could stand in front of it and watch the woofer go in or out as I connected a battery to it. I had never done that test before. Once the plate amplifier is connected, you can no longer do such an easy test. If you try to use a polarity matching app as I have done, you quickly find it is not reliable and even probably wrong at subwoofer frequencies. This is because room reflections become part of the mix before you've even had one "cycle" at low frequencies.
That right sub has a polarity menu item, and it is now set to "Normal" (it had been "Inverted" for some unknown previous amount of time, perhaps since I did the plate amplifier replacement 2 years ago). Right now I don't remember why I set it to Inverted or perhaps it was a mistake.
The left sub, OTOH, has its polarity inverted because (so it seems) I installed the replacement plate amplifier incorrectly. I knew this was the case the first time I replaced the plate amplifier, so I ordered a polarity reversing XLR cable to connect it with, rather than having to take the plate amplifier out all over again (which requires moving a lot of stuff around). That was about 12 years ago. I used the polarity reversing cable on the left for awhile, but at some point (while I was still using Behringer DCX 2496 as crossovers, and they have polarity control) I decided on a trick: I'd reverse the polarity on the left and then reverse the polarity to both subs, making them both correct.
In 2013 I started using Behringer DEQ 2496 as crossovers (because they have a digital output option, allowing me to use high end DACs) but they do not have any polarity control (though it seems now that older ones are polarity inverting), and then in 2021 I started using miniDSP's as crossovers. Somehow in all these changes, I lost track of the fact that the left sub polarity needed to be inverted (as with the inverting cable) to be in non-inverting polarity. I also replaced the left sub plate amp a second time, but I think I may have made the same mistake as the first time.
The problem I faced recently had nothing to do with absolute polarity (which, for subs, I now think isn't even an issue at all) but with the relative polarity between the panels and the subs.
With the 24db/octave filter, the top or bottom polarities can be reversed without having any effect on the frequency response. Linkwitz pointed out that the "inverted" phase connection with the 24dB/octave filter has the lowest group delay, but I've always believed in having everything in non-inverted polarity simply because it's possible and more intuitively nicer.
(Although it's possible that at some point I decided to use the inverted sub polarity instead. It should have had no effect on frequency response.)
I was thinking he 48dB/octave Phase Linear Linkwitz-Riley crossover I am using now has that same property, you can connect the drivers either way and get the same frequency response. However, I now know that is not true, it makes some difference which way you do it and you get some cancellation at the crossover frequency doing it incorrectly. So it actually seems that the "Phase Linear" FIR filters I am using don't have the same property in this regard as the normal Minimum Phase LR filters. Perhaps it's because they don't have any phase shift at all, so the phases are either matching or not, there is no in-between.
Mind you, the crossover is so steep the "cancellation" (which isn't perfect either) is confined to a very narrow band around the crossover frequency, and can mostly be masked with EQ (as I had been doing without knowing it) mostly.
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