The fact of the matter is that when it comes to group delay some will say it doesn't mean anything (asaverfi), and some will say it does (me). In reality if group delay really went hand in hand with type of box used (ported, sealed, passive radiator), then why are there sealed subs out there with worse group delay measurements than ported ones. Now you always hear about people preferring sealed subs for music than ported because they find them to be more accurate and punchy. As you can see that is not always the case as a sealed sub can have just as high of a group delay than ported ones. But in the end it is up to you to decide if it really matters to you or not. All I know is that the subs with some of the lowest group delays (Rhythmic Servo 12, JL F112) are the ones that most people always refer to as being very musical. I for one believe that group delay is the closest thing we have in order to measure accuracy and speed of a sub so I always care about that measurement.
I have no interest in endlessly debating this topic. There are perceptual papers that address the audibility of group delay while not definitive at such low frequencies the seemingly accepted value for group delay audibility (which in all honest seems fairly stringent) is one cycle. Virtually all subwoofers have a group delay within this range, especially at the price points concerned within this thread.
As I previously mentioned in post 24 group delay is correlated to frequency response. The natural musicality found in the subwoofers you listed can be attributed directly to their frequency response and natural roll off slope. Such aspects can be easily recreated via proper use of equalization.
Let us take, for example, the SVS SB12-Plus and SVS PB12-NSD discussed previously in this thread.
Frequency response is shown followed by group delay:
If the group delay is compared between the two units it certainly seems as if the ported box has a superior measurement in certain areas, specifically 30Hz-50Hz range. Now if the frequency response plots are analyzed at these points it is clear the ported box has a more flat response in this region. This flatter response directly effects phase which is where group delay is derived. The group delay itself plays no part in the perception of musicality it is just a poor, indirect, measure of frequency response.
The reason ported subs often, not always, have higher group delay than sealed subs is because of their extended bass shelf and different natural roll off relative to sealed subwoofers. These factors also contribute to perceived boomy nature of sealed subs (although likely not as much as room interaction) and can be mitigated through proper use of a single shelving filter.
