The main design goal of the JBL speakers is for professional sound mixers for films and music can continuously listen to 85 dB levels and up to 115 dB peaks at about 20 to 25 ft away, with low distortion. I think this is around 130 dB/W/m efficiency. They had to go to extraordinary lengths to get a tweeter and 15 inch woofer to play nice together. In other words, just to get the off axis response to match the on axis response pretty closely out as far as 60 degrees off axis. You can listen to the designers of the speaker talk about the specifics on the somewhat recent Home Theater Geeks episode (
https://www.youtube.com/watch?v=CDmzfpf3fCk ). I found the podcast episode very interesting. I loved hearing about all of this new and exciting design approaches that went into making this speaker.
For most typical consumers we do not need this speaker. Ascend already designs loudspeakers that are accurate with exceptional off axis response, excellent transient response and many other attributes. As long as you are not wanting to play them at 85dB continuously with 115 dB peaks at 20 to 25 ft away, the Ascend towers with the Raal ribbon tweeter might even outperform the JBLs in some areas. Do you need speakers that can play this loud? Even Scott Wilkenson, the host of the podcast, asked the JBL designers several times why recording engineers want to listen to the speakers at such a high level for extended periods of time. His concern was damage to their hearing. If I had a home theater that I was setting up for 20 or 30 people I might seriously consider the JBLs. Realistically, I might setup a home theater for at most two rows of seats where the second row is at most 15 feet away from the speakers. The Sierra towers should give more output that I would ever want or need in that case. In fact, the bigger challenge for a multiple row setup from my perspective is getting the sight-lines right so that all the viewers can see the screen unobstructed. But, I like a really big screen for an immersive experience. This is why I am likely just going to stay with one row of seats in my home theater.
Anyways, the reason why Golden ear, SVS, etc. are putting these "subwoofers" in their cabinets is to appeal to the audiophile crowd that thinks subwoofers are bad. The only speakers that this crowd wants in their room are full range large tower speakers for a two channel setup. In just about any room, if you were to cut off the lower portion of those cabinets to give you the freedom to move the subwoofers around in the room to find the best location for uniform bass (midpoints of opposing walls in sealed rectangular rooms) and put the top portion, the satellite speakers, where you can get the best imaging, it would be possible to get the speakers to sound good in more than just one seat. In fact, with a bit of EQ on the subs, you can likely make it sound better in several seats than you could in one seat leaving the speaker together as a tower and no EQ. I am mentioning this because the audiophile crowd doesn't even like much manipulating the sound with things like EQ either, to help correct anomalies that arise from the room interaction.
I should mention that having tower speakers in a larger space can be of use to help pressurize the space. A recent article goes into much detail with this:
https://www.audioholics.com/room-aco...ation-of-sound. So speakers like the SVS towers, Golden Ear towers, etc. can be of use from this perspective. However, the Sierra towers should be sufficient in most spaces, even ones that are quite large, in terms of helping the subwoofer pressurize the space. But, I doubt the SVS towers, or Golden Ear towers and the like, were originally designed with this in mind. I do think that they are more likely trying to cater to the audiophile crowd that doesn't like subwoofers and plan to use them in a two channel setup.