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I'm interested in your first listening impressions. Are you high-passing them or running them full range—could you give some crossover details for both the Cubes and the bass driver, please?
Your assumption is off. The 98dB you refer to is average sensitivity across the woofer's usable response, and that driver is designed to play higher than most. Adding a 6mH coil and 48uF cap (Butterworth alignment) will likely result in output closer to 88dB @ 300Hz and significantly less below that. I personally think your starting point of 300Hz/2nd order electrical is too high for what you are looking to accomplish. An all passive design like this would likely benefit from a steeper/lower xo, which would further necessitate the FC8's attenuation. There are ways to accomplish this without seriously degrading the FC8's SQ.
Great. Just a word of caution about the Altecs being driven by a 1000 watt class-D amp. Popular Altec woofers like the 416 and 515 used in VOTT applications were essentially woofers designed for horn enclosures (often in multiples). Due to the enclosure, they were very efficient in their day and driven by low powered amplifiers, even in movie theatre settings. Because they are not high excursion modern-day woofers, they may lack the geometry necessary for open baffles... depending on your listening level. At the same time they are also very stiff. Because of this, you may notice very little cone movement when they play... but are still subject to damage with excessive power. Their strength is in their ability to play higher than most traditional 15" woofers. They were often crossed at 800Hz, which would not coincide with your stated goals.
Curious about how these speakers made out for you. Still using as OB? Any changes to your set-up?
Why not drive the woofers with a separate amp and a low pass filter...?Or do you feel like you need a higher quality woofer to match the more expensive full range speakers you have now..?