Addressing the original post, you bring up the ultimate question, IMO.

So many posts regarding 'seamless' sub integration overlook the fact that unless your satellites are 12 dB/octave (sealed), you will not be able to acheive a 'seamless blend' using standard BM.

I think the answer to the question raised is that 'seamless' integration of sub/sat at 80 Hz requires the sat to be flat (+/- 3 dB) to 40 Hz, and, yes, it's because there are very few, if any sats available that can meet this requirement.

The problem with 'large' sats is that, as most people know, having, in effect, 5 or 6 subwoofers that have little, if any, placement options (for example, corner loading a full range speaker is a disaster), makes this setup less than optimal.

Stereo subs still requires that you match the high pass filter/sat speaker type to the low pass filter slope on the sub.

The road to the answer is a simple one: Use a low pass filter slope of 36 dB/octave on the sub when using ported sats with a 12 dB/octave high pass slope. Why this isn't a standard option just baffles me.

The next step is to have a dedicated LFE sub system to keep the LFE (mono) signal out of the sub/sat integration scheme.

Stereo has no LFE, MC music uses the LFE as a bass boost channel with little to some signal and movie soundtracks use huge amounts of LFE + 10 dB signal. The only way to control it properly, as to calibration and integration, is to keep it discrete through playback, with it's own LP point, slope and phase/time adjustment options.

Then again, I could be completely mistaken.