I spoke in general terms using the word(s) "usually" and "in general", I'm sure there are exceptions. I looked at a few specs from major manufacturers for SPL speakers like Sundown and most products had ratings below 90 db, the SQ speakers like Blues Audio had rating above 90 db,two very different types of drivers. You can make your own conclusion, maybe I misunderstood. This is a part of a conversation I had with a president of a SQ speaker company:
"Most subs today waste a lot of power because the suspension is so heavy and their efficient so low (81-85 dB Efficiency). They do that to rate them a high power handling figure which really doesn't tell anything about the musical output quality or volume level. Because a speaker electrically handles a quadrillion watts doesn't mean it acoustically produces a lot of sound. They have to have super big magnet/motor structures and a super tight suspension and heavy moving mass they throw away a lot of the power in just moving it. An 88 dB efficient speaker will produce the same SPL level with half the power as a 85 DB speaker. A 91 dB speaker takes 25 percent of the power of an 85 dB speaker to make the same volume level with the same given amount of power.
Most subs today are even less efficient than 85 dB, and 82 dB speaker will take 8 times the power to get to the same level as that 91 dB speaker. Efficiency is very important so is the Fs of the driver. That basically tells you how low it will play. Most of these heavy weight SPL woofers don't even play good sub bass. The Fs is high and their compliance low which means the take a big big box, and a lot of amp power to make sound and they are basically tuned to produce sound in the 60-90hz range which isn't really a sub woofer that's just a woofer. Hell our 6.5's play lower than a lot of 10-15 inch woofers."