Dazzdax, to get back to your question....Driver manufacturers have to balance efficiency, durability, linearity, smoothness, and frequency response, at least, to provide an excellent driver. Limiting factors is smaller two-ways are almost always the bass roll-off of the smallish woofer in its chosen bass alignment in a given volume (with or without vent, etc.). Usually the result is a rising frequency response unless crossover components are used to roll the mid/woofer off. Almost always tweeter sensitivity is higher than the mid/woofers used, so the tweeter must be "padded", usually with a resistor, to bring the response down to the desired spectral tilt. Lots of tweeters are left pretty "hot", resulting in that "tipped up" or "bright" response of some manufacturers' designs. This can also make it a bit easier to slightly inflate an efficiency spec, or at least provide a louder response with a given input signal. Other designers, seeking a warmer response, will pad the tweeters more, resulting in a fatter perceived bass, but resulting in lower overall efficiency, and the requirement of turningthe gain up to achieve a desired loudness.
Balancing bass response and extension with a chosen upper octave spectral tilt is the meat and potatoes of this, not power absorbed by simple crossovers, nor tweeter resistors, as they're operative above crossover frequency.
Indeed, adding MORE drivers, all other things equal, will increase efficiency nicely, just like using a bigger amp. ence the use of 3-way designs or multiple woofers. So in a simple way, you can see that a formula is to use as many woofers as necessary to match the sensitivity of the tweeter, for max efficiency using these drivers. Since lots of OE tweeters have sensitivities in the 89-93dB/w range (I'm guessing), it's pretty easy to design speakers with highish efficiencies if you use enough woofers and/or box volume to get equivalent efficiency in the bottom octaves. Dorealize that voice coils get REALLY hot...with consequent non-linear response, changes in resistance, blah blah. If a driver manufacturer uses too small a voice coil gap to increase motor efficiency they risk insufficient heat-sinking so the driver blows up...or even scrapes as it hangs off-center! Huge ss amps have allowed woofer manufacturers to design relatively inefficient drivers that are durable. Problem is you need to push 'em to get a lot of bass response, or again use a big box, or multiple drivers, pushing up costs dramatically.
I look to the Spendor SC3 center channel speaker I use, as a simple example. It uses two 5" mid/woofs and a 3/4" tweeter, and sports 88dB efficiency, exactly matching the 6.5" two-way S3/1P's that comprise my HT front trio. I could have bought the SINGLE-woofer S3 (old LS3/5) versions, but these have damped tweeters squashed down to match the much lower sensitivity of using only one woofer, and thus spec at 84dB or so (there are vent and box volume diffs too, the efficiency difference isn't exactly algebraic). You might have asked why the same-tweetered speakers have such a different rated efficiency (sensitivity). Now you know. Hope this helps.
