If you add enough magnet you can reach Qts 0,19. But that's is not relevant here, this is just a demonstration to see what happens if the driver Qts is lower.
Where do you get that Genelec and Neumann use low q drivers, they are not for sale for the public and are in house designs, not variations of drivers we know, even if they are made in factories where drivers we know are made, they are not the same, they are OEM productions of own designs from the companies, tuned to their wishes. And both use extensive processing in their speakers.
Okay, I just guessed that they use low Q drivers. But even if they don't, their monitors low-end responses are high Q (which is similar to a low Q driver in a vented box) with the resulting relative high GD. The Neumann KH420's woofer is a PHL woofer even if not a stock model. Genelec also uses PHL woofers in its larger monitors.
Genelec 8831A group delay, more than 60 ms around Fb, of course there is probably also a protection high-pass filter in it but this is the result:
https://www.erinsaudiocorner.com/loudspeakers/genelec_8331a/
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Here is a pic of the Neumann KH420 woofer, it's probably a PHL 3411 with an added (shielding?) magnet:
And the stock PHL 3411:
And the stock PHL 3411:
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Those brands use OEM companies to build drivers, but design them largely themselves to their own specs. PHL does build drivers for Neumann, but they are by far not standard. They may use the standard frame (what is not so important and magnet cover, but you can see that the Neuman woofer has a different cone, that is way more shallow than the PHL you mention. The magnet can also be very different, even if the housing is the same. It's not because a driver looks like an other driver that it's the same.
Genelec does the same, they design and specify a driver and find an OEM builder to do the production for them. They don't use standard drivers in most cases. For both designs you can have the result they have with a low Q driver in their cabinets, you need a higher q and i'm sure those they use have a higher Q. Genelec uses also a lot of Seas or Audio Technology made drivers btw, all custom and not availeble for the general public. OEM is the core bussiness for those brands, and their diy offfering is often more advertising what they can do, not their main production.
Genelec does the same, they design and specify a driver and find an OEM builder to do the production for them. They don't use standard drivers in most cases. For both designs you can have the result they have with a low Q driver in their cabinets, you need a higher q and i'm sure those they use have a higher Q. Genelec uses also a lot of Seas or Audio Technology made drivers btw, all custom and not availeble for the general public. OEM is the core bussiness for those brands, and their diy offfering is often more advertising what they can do, not their main production.
The magnet is the same (even the pole plates seems identical), except the added magnet on the KH420 woofer. Why would they used a similar large magnet as the 3411 have, if they don't want a low Q that the 3411 have? Edit: okay, maybe for compensating a higher moving mass.
Anyway, if you check the low-end response of the Neumann KH420 carefully, you can see the ripple around Fb before the rolloff. This usually comes from a low Q driver (in a vented box) or a protection high-pass filter with high Q or the combination of these two. The end result is a relative high GD which don't seem to bother the manufacturer or the listeners of the KH420.
Edit: Okay, I asked Neumann about the Qts of the KH420 woofer. Will see if they answer.
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Here is a sim with specs from one of my real prototype low Q subwoofers against a theoretical copy with just BL dropped to raise the qts and the rest of the parameters verified as compatible through winisd. In the same enclosure, same airspace, same port dimensions, the tuning frequency will be different beween the two, but a you can see peak group delay at tuning is still higher on the higher qts sub.
So, like I said: Two subs are in identical ported enclosures, with the only difference between the two subs being BL/qts, the lower Q sub will have lower peak group delay.
So, like I said: Two subs are in identical ported enclosures, with the only difference between the two subs being BL/qts, the lower Q sub will have lower peak group delay.
Okay, I see what you meant and of course you are right but so I was. Your simulation is pretty extreme, the Qts is so high and the Vb is so small (compared to Vas) so this box results in peaking in its frequency response higher than the Fb and this is why it have a higher group delay peak.
So as @KSTR also said, the group delay follows the frequency response and in my case, the GD peak was higher for the lower Qts driver because it had peak around Fb and in your csase, the GD peak is in higher than the Fb because the motor of the driver is so weak (and Vb is so small compared to Vas) small it can't push the air enough at the port tuning, causing peaking higher in frequency, hence higher GD.,
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If I am interpreting you correctly, in the end all you have is FR, most likely. The shape of FR can be translated into Phase/GD if I am not mistaken. So your concern is of a shape of FR, roll off in particular, and the bottom line is the desire for low Q, is to have a woofer that is in control. GD/Decay as a result of FR created by driver + enclosure is sort of a separate topic. Fix FR and you will fix GD. Raising Qts will not give you a woofer that is more in control, but the opposite.