I suspect that computer modeling has made a huge impact on driver design. They can now accurately predict breakup and non pistonic behavior, and design even low cost drivers to minimize these aberrations.
Though I don't have any real knowledge of mid fi speakers, I imagine they have gained as much in quality as true high end products, simply by virtue of being able to buy or manufacture these modeled drivers, and know where their cheap boxes will flex and need reinforcement.
Just look at some of the sub $1000 models being reviewed in TAS and Stereophile. They get great reviews. I dont think this point would have garnered serious listening or many reviews 10 years ago.
I use quite small powered genelec monitors in my mac based edit and mix system. The shape of the metal cabinet and built in wave guide was all computer modeled, with a corresponding sound quality.
Quads and Soundlabs (with new PX panels) have improved immensely over the past decade, playing lower, louder and more dynamically.
Better quality caps are available for crossovers, and better internal wiring as well.
More and more designs seem to shave away more of the front baffle for less diffraction.
Cabinet materials have evolved as well. My current speakers are made from ceramic wrapped in kevlar.
No mater what choice you make, familiarize yourself with recent offerings. If nothing else it's fun and informative.