I would agree with this to an extent, that a flat response should sound the same regardless of the sub, but you aren’t taking other factors into consideration. 2 18s playing at 110db will sound better and less strained than 1 12 playing at 110db, even if they both measure flat. Likewise I tend to believe that higher efficiency pro drivers, sound better in the mid bass region, as they suffer less from thermal compression and take less power to get to the same spl, again reducing distortion, which further translates to better dynamics and transients or ‘tightness’ (a word which I don’t tend to use as it offers no real insight into what is being described). It has been linked above, using the example of PSA, that higher efficiency pro drivers sound cleaner than high excursion drivers, even if they are both EQ’d to the same response curve.
I don't disagree. I was definitely way too simplistic with my statement and should definitely have included phase/group delay and THD. That said I'm not so confident that it makes all that much difference in reality. Don't get me wrong, they DO make a difference but I'm wondering how much of a difference they make these days as there's a line I'm not sure you ever cross. That line defines differences you can hear and ones you can't.
As I said in an amp thread, you CAN make some horrendous choices and make things accidentally (or even purposely) sound different but generally that's not the goal. In a sub if you want very high SPL at a low frequency then you don't chose a single 12" driver. Of course you CAN do it but why on earth would you do so out of choice?
In your example, if you want 110dB you wouldn't, normally, choose a 12" driver or small enclosure as that's not going to get you to 110dB down low. Conversley if you did, you'd have to accept the comprmises that limited space comes with. So sure, a single 12 is absolutely not going to sound as good as dual 18's down low at 110dB's
Now on the other hand, if you were to compare 110dB on a dual 18" over a single 21" measured flat in enclosures designed to match the speakers parameters then I'd start scratching my chin and doubting things. I doubt, in use, there will be any audible difference between them. Could you get an audible difference? Sure, just making a cabinet too small of constructing it badly will do that for you. Or make the ports the wrong size. Or any one of a hundred things you could do to make that difference but in reality you never would.
But moving away from the theoretical the reality is you'd stand on the shoulders of giants and you'd most likely build one of the very well designed "DIY" subs using one of the well designed for purpose speakers. For the vast majority, they don't know or really care because frankly unless you're going to go into it with the sort of depth
@mattkhan goes into it (and he REALLY does know his stuff) most people just want to build a great performing box cheap (or at least much cheaper than a commercial sub or equivalent performance would). Those designs, generally, don't suffer from any of these problems.
With the internet the way it is, the free and easily available wealth of information and the ability to get plans, designs and builds that have been designed by, in many cases, experts in the field I think we are broadly past "bad" designs and builds. That's not to say designs can't be improved it's to say that on the whole we've never been in a better place with audio where even the entry level really is exceptional high and higher than most people can appreciate.
As I said I don't obsess over this but it's an interesting discussion.
G