Do you think Def Tech's drivers & DSP tuning are responsible for its middling performance numbers? Surely if they added a 1000W amp it would've brought much greater headroom in the mid-bass region like REL's HT/1510 Predator II, there is absolutely no excuse for them to charge $1799 only to include a 500W amp, it's understandable for their smaller DN12 12" variant to have 500W like REL's 1205 MkII or the Monoprice M-12 V2 but the other companies had the smart sense to employ more powerful 1000W amplification for their 15" counterparts, it truly is amazing that the Monoprice SW-15 @$300 with a 600W amp offers greater punch even if it isn't as refined sounding as the DN15.
You are recent member, and welcome. We cover this often. There are hard and fast laws of physics with audio reproduction and that is especially crucial in considering subs.
First is that amp power can not be considered in isolation. A sub driver with 3db higher sensitivity will have the same acoustic output with 500 watts as one with 3db lower sensitivity which will require a 1000 watt amp for the same output.
So, you may ask why are all subs not designed with high sensitivity? The reason is that there is an inverse relationship between sensitivity and bass extension. That is why pro subs used in cinemas and live theater performances have relatively low bass extension as sensitivity is paramount.
How the sub is loaded makes a difference, in general the bass output is highests for horns, then pipes and ported/passive radiator close seconds. Sealed are way at the bottom of the heap, and will have native f3 in the 40 to 60 Hz range, and require a 12db per octave boost below their native F3. That amounts to a massive acoustic efficiency penalty, and why they require such monstrously powerful amps. One other point is that a sub will not go significantly lower than the driver fundamental resonance known as Fs. Only sealed subs can be boosted below F3 that is the 3db drop in output point. The reason is that the driver decouples from the loading of the enclosure abruptly in other designs. So response falls off like a rock.
The take home is that you can have enormous bass output from an efficient design with comparatively little power. That is true in my approach, but the enclosures are very large.
As far as high frequency extension that is not usually of great concern in sub designs. That is because subs are often placed away from the main speakers and you don't want localization. However there are other issues. As Shady pointed out, heavy cones, which tend to a lower Fs, are not going to have much top extension as a rule. Even more to the point big heavy cones are likely to break up, which means the cone does not move as an entity. These break up modes result in nasty frequency peaks which you don't want, and require higher frequencies to be smartly rolled off.
However, you do have a point about the power of music for a couple of octaves above where subs are crossed over. You are correct if you are trying to point out there is a lot of power in that range. To make matters worse it is where speakers with smaller drivers and narrow fronts will be transiting from monopole to omniole. This is known as the baffle step response and has to be equalized for, with a 6 db boost below that transition frequency.
To compound the problem you can't really test speakers like you can other components like amps, where you can drive with as much power as the amp can take and measure the distortion. Speakers on the other hand have wildly different power and output capabilities at various frequencies, which are always totally unknown except from careful listening. In my view this is the largest confounding factor in choosing a speaker system and you won't get much clue of that aspect from standard speaker measurements.
When I design, I look carefully at the frequency power ranges of the music I listen to, which is pretty much classic, opera and choral music. This program material has massive power demands below 1500 Hz down to 80 Hz. Yes, you have it right, above sub range.
So in my main system, I don't actually use sub drivers, but four powerful 10" SEAS Excel drivers, with an Fs of 20 Hz. With active drivers I can also supplement the mid drivers below the transition frequency. The design is integrated in the main speakers, to make seamless full range speakers, with no sub in the traditional sense.
But that results in room spl. than you could ever use across the whole frequency spectrum. The system uses modified stopped pipes known as transmission lines. The system is not commercially viable due to size, and what would be enormous production costs. But is gives me a powerful unique system.
