I'm wondering if building new 10cf cabs for the W12s would be worth it for an extra 10Hz output down to 30 Hz.
My opinion would be no, it is not worth building 10cf ported cabinets. You are using active crossover already, so you should seal the boxes and apply boost to get the low frequency response you desire. It will sound better.
The driver can take 300W peak during music playback, and are 90dB/W/m. So they cannot play 110dB at 30Hz without breaking. If you want that kind of dynamic headroom at low frequencies, you need 18+ inch driver and a big ported box, or sealed with 1000+ watts.
HiVi recommends sealed box of 75liters per driver. That's 5.3 cu ft for two drivers in same box. Sealed volume doesn't have to be exact. Try what you have now, you can adjust it with the crossover.
http://www.swanspeaker.com/asp/product/htm/view.asp?id=111To answer your original question, there is not a lot of musical information below 40Hz, that's the lowest note on a bass violin or bass guitar. But there is a lot of information about the acoustic space below that, and there is musical information in bass drums, and organs if you listen to that. 40Hz+ is enough for most people, it is very low, especially if only -3dB at 43, that is pretty loud.