What you said is absolutely true, but (and you know this, I'm sure, but I'm just stating it for the newer folks), if a subwoofer is going to be used for home theater duty, you may end up with different recommendations since HT duty can place additional requirements for infrasonics and output levels that may not be as important for music listening.
Before you decide to go the "minimal" LF extension route, this may be of interest:
In the music studio our main monitors only go down to 35. That's enough for music, indeed.
But:
The room is big and optimized and almost flat.
In contrast my HT-living room, due to its smaller size, its almost squared shape, and for obvious reasons of not unlimited acoustic treatment possibilities, has a peak at 58 and a significant hole around the 80 area.
When I built my home setup, I was hugely surprised that a subwoofer going down to 30 was not enough for
music once the 58/116 resonances were killed...
Boney M - Daddy Cool (Greatest Hits of all Times Remix Album), which I like, because it is extremely weak on the sub bass and probably only has >65 Hz PA kickbass. Sounds full enough in the studio, on PA in clubs, but revealed problems in my smaller room: ruthlessly, once the 58 roommode was killed, the kickbass became annoying. After kiling the 120 mode the mix had not enough body (1970s disco did often not even go down to 50).
After several subwoofer tests with many measurements, I decided for subs that slightly go down below 20!
The difference due to the LF extension from 30 to 20 is significant.
First it did not make any sense to me. How can the extension down to 20 have such a positive impact on the balance of the mix, if the mix does not contain significant LF at all? That's stupid, isn't it?
I can only speculate:
First a sub that honestly goes down to 20 is easily reproducing 25 or 30, but a sub that "only" goes down to 30 already has -3 dB @30. And 30 is already a region, where there might be a small amounts of sub bass being left in the mix, which the 20-sub is reproducing and may create that effect.
There might also by a kind psychoacoustic effect at work:
Just like the amount of treble influences our bass perception. Our hearing is not working like a mic or a spectrogram. (Gladly) it is possible to fool it. It allows music reproduction and to use shortcuts, or workarounds, if physical restrictions would not allow to apply the physically correct solution.
Long story short: LF extension has worked very well here. It is a great sounding solution for music, especially in rooms, where cancellations in the bass range are unavoidable. So instead of trying to chase big space PA-like kick bass in small rooms, which is very fatigueing, I think extending the very low end, is the way to go.
think about it: how long will you have the sub(s) if you are satisfied? 10 years? 20 years?
What is the price difference from a 30 Hz to a honest 20 Hz sub? How much is that per year?
And last but not least:
If you choose a setup that goes down to 20, and at some later time, if you notice the fatigueing effect of loud 80-150 Hz mixes in the too small room, you can reduce that range and bump the 20 range. But if your sub only goes down to 30, or even 35, then you cannot create this kind of "drop octave" effect.
