Full range, quality high efficiency speakers tend to be big and expensive.I think you have blame the laws of physics for that!!!
NEwer high efficiency amplifier technology is capable of taking a lot of the technical guesswork out of the equation and driving most ANY speaker well. PLus they are small and affordable!I assume you are talking about Class D or switching amps. I'd love to hear Atmasphere and Almarg comment about how that type of amplifier's inherent electrical characteristics interacts w speaker loads.
W respect to the 'why" of inefficient speakers w widely varying impedance and phase angle v. frequency response, it's my understanding that a lot of that has to do w the laws of physics. You need a large driver to propagate bass energy. But large drivers tend not to work very well at higher frequencies. So in many cases you solve that w a multi-driver speaker w a crossover. If you want extended HF & LF response and have the important mid-range covered by a single unit, you end up w a 3 or 4 way design w a more complex crossover. To maximize efficiency in the bass region (which really eats up power), many designers use ported speakers. All of these elements make it difficult (not impossible) to design a full range or near full range speaker w flat impedance and modest phase angles. If you've been following other threads, you will see that there is market resistance to so-called "simple designs" at what some perceive as "high" prices. And then there is the "wow" factor of those multi-driver behemoths like the guy from "New Hampsha" hawks. Pay $10K for a near full range 2 way or $30K for a near full range single driver or pay less for a 7' tall coffin w 5, 6, or 7 drivers. You make the call.