The 2.5-way crossover design explained
A 2.5-way crossover gets its name because it behaves like a hybrid between a
2-way (woofer-tweeter) and 3-way (woofer-midrange-tweeter).
A woofer mounted on a baffle exhibits increasing output in the midrange with increasing frequency. Normally this rising response must be tilted back to level by the crossover, and is cause for about 6dB loss in sensitivity. This loss can be recovered by the 6dB higher sensitivity of 2-woofer designs, such as with the MTM layout. MTM speakers employ a single 2-way network.
A 2.5-way crossover gets its name because it behaves like a hybrid between a 2-way (woofer-tweeter) and 3-way (woofer-midrange-tweeter). In fact, the crossover inside a 2.5-way speaker has three distinct sections, similar to a 3-way network. Put another way, the name also implies a regular 2-way speaker supplemented by an additional '.5-way' woofer. The high woofer crosses over to the tweeter like a regular 2-way, but the '.5' low woofer is gradually tapered out beginning at a much lower frequency.
In this fashion, the high woofer's naturally rising response is intentionally not tilted back to level. Instead, the low woofer is used to fill in the lower half of the midrange, where the high woofer is lacking. Acoustically, the two woofers sum similar to a first order crossover. You can view this behavior at the Series 2 TMM SPL graph.
So what's the big deal? Well, since just one woofer reproduces the upper midrange and low treble, there is no comb filtering as with a regular MTM. The dispersion pattern is essentially that of a single woofer MT layout. Subjectively the sound stage is more spacious. Finally, a common dual-woofer speaker can be used for left-center-right, and with a common voice to all listeners.