While the tippy-top in terms of subwoofer driver design is maybe only a little better than ~10 years ago (I'm thinking of drivers such as the JBL 2235H, McCauley 6174, Aura NRT-1808, and Avatar/Adire Tumult Mk. II, and the TC Sounds OEM'ed Audio-X-Stream and Crystal Sounds underhung drivers, with the Avatar/Adire Shiva and NHT 1259 being the budget stars of the time) there have been some noteworthy changes since then.
1) More convergence towards best-practices design. A decade ago, the typical "high end" subwoofer driver was likely to have a stamped steel frame with no under-spider ventilation (all of the drivers above except the Shiva had cast frames, and the Aura had under-spider ventilation), no Faraday rings in the motor to control inductance, and relatively short throw. Since drivers such as the Peerless XLS line (probably a bit less than a decade old, but not much far removed from that point) started pointing the way, more drivers have moved towards more sophisticated motors with shorting rings and low inductance, along with cleaner suspension designs.
2) Lower cost for outstanding performance. A decade ago, performance very near today's SOTA was available. Today, performance at that level costs much, much less. Thanks, China Price!
And along with better, cheaper drivers, there are three other advances that endow subwoofer systems
with much greater potential for high-fidelity bass reproduction, in what I consider order of importance (the better drivers would be #4 on the below list):
1) Computer-based measurement solutions. A decade ago, good
measurements were hard to come by, because tools were so crude. The typical serious audio hobbyist had something like a Rat Shack analog SPL meter and a test disk such as "Sound Check with Alan Parsons and Stephen Court" that had test tones on it (Mobile Fidelity SPCD-15). Now. a serious audio hobbyist has a calibrated measurement mic (or several, because spatial averaging is a better way to take measurements in situ) and a program such as FuzzMeasure Pro to take in-room MLS measurements.
2) DSP equalization. A decade ago, there was crude and expensive analog equalization available through parts such as the Marchand Bassis and AudioControl Richter Scale. Today, there are plenty of digital solutions available that are more flexible and often cheaper than the analog ones, or even integrated into amplifiers. And that's just to tailor low-end response. Systems such as Audyssey MultEQ XT also correct response within the passband, for higher-fidelity reproduction.
3) Wider availability of "novel" amp designs such as Class D, Class H, and Class G. These newer, lighter designs (especially when coupled with cheaper, lighter switch-mode power supplies) allow more power with higher efficiency and lower cost.
Originally Posted by audioguy
I see that very differently. Could a sub, ten years ago, produce 115 db at 20 Hz (or lower) in a home theater with reasonable distortion in a box smaller than a large refrigerator and costing less than a large SUV?
Yes. A driver such as the Adire Tumult with a Crown K2 could have gotten you there back then in a reasonably-sized box. (In commercial subs, maybe 2-4 of the NHT subs with their 1259 driver, assuming it was used outside of the Model 3.3?) But equivalent performance is much cheaper now.