Looking for a new Subwoofer

i would go w GR research dual open baffle servo subs either a DIY kit if you have build skills or already made 
would be far better for the $ than ANY you mentioned ! Vandersteen? OMG those are crap ! 
if you want to go more old school watch some of the audioholics YT videos about subs they suggest combining sealed and ported subs one of each 
 

multiple subwoofer home theaters. The benefits include:

► more even bass distribution across the seating area

► better seat to seat consistency for more effective EQ

► more output and LF coupling factor for deep extended bass response

Ever wonder why some AV receivers have only one subwoofer output while others have two or more? How do you determine if the dual subwoofer outputs are independent or parallel and why does that matter? We discuss all of this in this video and show you with measurements why it's so important to get the proper time alignment in a multi-sub system.

Multi-Subwoofer Set Up & Calibration

•Choose your subs wisely-preferably all identical subs or subs with near equal f3 (3dB rolloff) points

•Place your subs wisely-in home theater rooms, nothing is more important for achieving good bass than proper placement of your subwoofers and listening seats

Multi-Subwoofer Set Up & Calibration

•Make sure all your subs are playing the identical signal -all of your subs should be playing a mono signal that consists of summed bass from all speakers set to "Small" plus LFE info

•Setup bass management-in your A/V processor/receiver, set all speakers to small, use 80Hz crossover setting as a start, and defeat the internal LPF of all of your subwoofers

the common question of whether or not you can mix sealed and ported subs into your home theater system and get good bass. I discuss the problems in doing this and how to overcome them to ensure you get good bass integration and avoid losing very low frequency response due to phase cancellations. 

Many good YT videos on the topic!

https://youtu.be/xo2xkjJ_-zk?si=iiUQTqK81djMtHaB

https://youtu.be/w1LG1m1tR8w?si=a2vvxTyS46b6vxQk

https://youtu.be/ypQu4FncAjk?si=t7sXlonDb-dLpBrY

https://youtu.be/8XE1JsfqKtA?si=pjR9cPiF-RJXvXBz

https://youtu.be/CbAO4-F5Ejo?si=erIyC3pXTW-ea0TP

https://www.youtube.com/live/qxKRinIaGnc?si=w6fHQrNid6jkI8D9

https://youtu.be/7ieX40Ktaxs?si=ZSNaLN4w_m8vdChN
GR research subs :

multiple subwoofer home theaters. The benefits include:

► more even bass distribution across the seating area

► better seat to seat consistency for more effective EQ

► more output and LF coupling factor for deep extended bass response

Ever wonder why some AV receivers have only one subwoofer output while others have two or more? How do you determine if the dual subwoofer outputs are independent or parallel and why does that matter? We discuss all of this in this video and show you with measurements why it's so important to get the proper time alignment in a multi-sub system.

Multi-Subwoofer Set Up & Calibration

•Choose your subs wisely-preferably all identical subs or subs with near equal f3 (3dB rolloff) points

•Place your subs wisely-in home theater rooms, nothing is more important for achieving good bass than proper placement of your subwoofers and listening seats

Multi-Subwoofer Set Up & Calibration

•Make sure all your subs are playing the identical signal -all of your subs should be playing a mono signal that consists of summed bass from all speakers set to "Small" plus LFE info

•Setup bass management-in your A/V processor/receiver, set all speakers to small, use 80Hz crossover setting as a start, and defeat the internal LPF of all of your subwoofers

the common question of whether or not you can mix sealed and ported subs into your home theater system and get good bass. I discuss the problems in doing this and how to overcome them to ensure you get good bass integration and avoid losing very low frequency response due to phase cancellations. 

Many good YT videos on the topic!

https://youtu.be/xo2xkjJ_-zk?si=iiUQTqK81djMtHaB

https://youtu.be/w1LG1m1tR8w?si=a2vvxTyS46b6vxQk

https://youtu.be/ypQu4FncAjk?si=t7sXlonDb-dLpBrY

https://youtu.be/8XE1JsfqKtA?si=pjR9cPiF-RJXvXBz

https://youtu.be/CbAO4-F5Ejo?si=erIyC3pXTW-ea0TP

https://www.youtube.com/live/qxKRinIaGnc?si=w6fHQrNid6jkI8D9

https://youtu.be/7ieX40Ktaxs?si=ZSNaLN4w_m8vdChN
https://gr-research.com/product/sub-series-double-trouble-with-flatpacks-copy/
https://youtu.be/1Q3QQPO7y04?si=pKYBmMzxtmBFkrU_
 

World Class Subwoofer Kits

Our Open Baffle Subwoofers use Direct Servo Technology and offer the quickest, cleanest bass that money can buy. There is no other subwoofer system that will give you this level of control and customization.
yea go GR research for subs i am .
 

 

According to the GR website, The GR kits are out of stock in addition to the completed single servo 15” sub. If they are not out of stock, the website needs to be updated. 

right on grissley 

Four cabinet Bass Array systems go low and provide an unparalleled room lock while using less power and EQ with minimal location restrictions.

Most if not all -3dB subwoofers require standing wave bass mode room (crawl test) positions to providing them the least restrictive path within the room allowing full advantage of their delicate extra low frequency presentation. These are typically side wall locations.

Another point to consider. Using one costlier processor equipped subwoofer as a master to control less expensive basic models or brands.

 

Any brand -6dB unit is a sub-bass speaker. Like the -3dB subwoofer they will augment your main speakers mid 30Hz region at the expense of the subwoofers delicate sub sonic presentation present on most recordings. 

Because of the -6dB sub-bass speakers dramatic low frequency roll off they simply do not excite a rooms standing wave bass modes. This gives them the convenient ability to be located most anywhere in the room with just a volume adjustment.

All the best with you search. 

As @soix (and others) have mentioned - you'll likely get more impact/improvement in your sound with a matching pair of subs. Enjoy the hunt, there is so much good gear out there.