Here’s the dual Fane 15XS bass bin 

It’s measured from 7 foot away, which is my normal listening position.

EQ is applied and both bins are playing in mono.

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I was recently using 38Hz-550 Hz crossover point for them.

The bass units are wired in parallel and achieve 104 dB/2.83V/1m. (4 ohms impedance)

Dual chambers are used inside. They have same the volume, with internal damping added.

Small amount of internal bracing, I purchased these enclosures used, and didn't actually build them.

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The null at ~200 Hz is floor reflection. Not much one can do about that. My floor panel (brown panel on seen on floor) barely gains me 3 dB improvement on that cancellation. Better than nothing, but not enough.

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Due to comb filtering (center to center distance of approx 20"), I have a null at 300 Hz. and 600 Hz.

This issue was expected, but I wanted more efficiency vs a single 15".

Having used a double 15" woofer set working in tandem, despite the comb filtering issue, I can state with absolute confidence that the advantages outweighs the drawback.

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The classic JBL 4350 monitor uses a similar bass unit which cuts @ 250 Hz to avoid comb filtering artifact, and retains the slam of a dual 15". However, JBL uses a 12" mid-range unit (250 Hz-1200 Hz) which brings plenty of other compromises compared to a compression driver and horn combo. The very low frequency is limited to maybe 35 Hz. No free lunch. Never is.

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I’m considering a 6" mid-bass front horn to experiment with.

I don’t think I’m ready for such big design change, but one never knows...