Getting good sound quality below 200Hz

Good suggestion, but I would still like to know of others listening experiences to just the frequencies below 200 Hz. Surely I'm not the only person to have done this.

I'd say that no, this is not necessarily typical of bass below 200 Hz. 

But, I think you might begin to get a more insightful frame of reference here, if you simply, say, try a different set of woofers to experiment with. Eminence woofers are relatively high sensitivity. That makes them good for lower powered amps, but high sensitivity woofers IME tend to not exactly plumb the bass depths and changing those woofers out for something like, say, Acoustic Elegance ob woofers (which are likely a bit more expensive) and you're at a sensitivity of around 90 or 91 dB. It may require a little more power than the Eminence, but you might experience a sizably different character of bass response in the room. One that may begin to show you just what a wide world of bass choices there exists out there to try out and to discover. 

By the time you've taken a crack or two at that, maybe then it may make more sense for you to resort to measuring to further your cause.

In the long run, I suspect ob bass will be better for you to get an overall satifying result than with boxed subs, but with a little more experience down that road, you might discover more reliably than now which you might really prefer, the clean, open ob sound that isn't as fully impactful, or the wallop of a good boxed sub that isn't really as transparent overall. 

Hope this helps.

Looking at the graph posted on https://eminence.com/products/alpha_15a it seems like it might not be possible.

Attack and snap In the bass relies on other frequencies above 200Hz. Are you overthinking this?

The baffles were bought with Altec 416B drivers and Saba Greencones, and the crossover point was set much higher (about 450Hz). This worked well until I upgraded the Sabas for Cube Audio FC8s. These were now capable of playing full range down to 200-250 Hz and this became the new crossover point. I did this as, in open baffle, the FC8s outperformed the Altecs in the range 200 to 450Hz. Having only to cover the frequencies below 200Hz was better suited to the Eminence drivers which were almost universally recommended for this use.

Out of interest I don’t play very loud 75db and the base driver cones barely move even though they give adequate volume,