Exciting project.
Lets stop talking about subwoofers and talk about this project as a new 3-way speaker system. Instead of thinking of adding a subwoofer and crossing around 40-80Hz I’m now thinking 150 - 300 Hz. [...]
As a fully active approach and using the same digital XO/DSP over the entire frequency span you’re dealing with, I certainly consider such a system to be a single x-way speaker system per channel - even when using separate subs (via the same active XO) crossed below, say, 100Hz. The main point here is that each speaker channel is treated as such through the active crossover (with on-the-fly settings adjustments via a control unit/PC/laptop from the listening position), and that you’re using a dedicated approach to both high- and low-passing (etc.) from the low frequencies on up, and not merely "latch on" active subs to an existing and not high-passed passive main speaker system - the by far most common approach.
Generally speaking I want to cross my new woofers as high as possible while still maintaining excellent off-axis response, not something a lot of commercial speakers have. In other words, I cut them off before they start to beam. Having the woofers located under the mid-woofers means I can cross higher without fear of the ear locating them, unlike a subwoofer appliance.
Maintaining a fairly flat power response is an important trait, not least in the main speaker driver transitions. Using waveguides or horns this can be achieved especially well crossing over to the woofer/mids, and at a lower XO frequency compared to using direct radiating dome tweeters. I typically don’t find the latter to blend well with woofer/mids below them, not least when compared to using waveguides or horns.
In the lower frequencies I always recommend placing a pair of subs symmetrically and close to the mains and coupled in stereo as a general rule, with the best case scenario being a fully active approach with high-passed mains and all. A mono’ed DBA has something else to offer, but while having an acoustically less smooth response (unless corrected with DRC) I usually prefer a dual sub setup as described.
By crossing high (150-300Hz) I reduce the load on the mid-woofers, increasing dynamic range but also has the extremely beneficial side-effect of reducing audible distortion even while playing low.
Indeed, but this is also dependent on the type of mid-woofers used. Using high efficiency and larger diameter pro segment woofer/mids (12" on up), not least in pairs per channel, they’re turned into rockets with plenty of headroom in a domestic environment when high-passed no higher than 80-100Hz. Crossing above ~150Hz is entering the power region (up to about ~450Hz), and this is also a sensitive area with regard to placing a crossover point, I find, and how affects the lower midrange and overall uniformity of presentation here. Oh, the compromises - both here and there.
