Sub suggestions please

There are some great suggestions in this thread but, not surprisingly, nobody is mentioning the exquisite Perlisten line.  Perlisten has a low profile on Audiogon, perhaps because the company rarely submits its products for review.  But  make no mistake, these subs are pretty incredible, deep low-end extension (12 Hz with built-in bass boost), effortless, transparent transient response, extraordinary detail.

They're not cheap, starting at around $2000, and don't offer a ton of bells and whistles, but they cover all the bases most important to audiophiles -- especially SQ.  IMHO, they're definitely worth at least seeking out. 

These are available in Walnut.  

https://gr-research.com/product/finished-sealed-servo-subwoofer-single/

I am not sure why the walnut finish is so important, but buying a sub that performs well would be my first priority and then add your own finish and get what you really want I would think adding some walnut veneer would not be that difficult. Time consuming maybe but not difficult. The SVS subs offer a lot of bang for the buck and their customer service is said to be pretty darn good.

I can recommend the SVS. You can buy and try for 45 days and return them for a full refund if you’re not satisfied. Their customer service is excellent. You can get on the phone with someone that will walk you through your setup and answer questions. The app is very easy to use. They do not come in walnut though so that may be a deal breaker. I also have experience with RSL and they are a good deal but no app unfortunately.

I think you should clarify some of your preferences.  I mean there are vast differences in subs. Movie or audio? Classical, rock, various? The speed of your existing woofers to match? Detail or impact? Do you want the sub to compliment your speakers in a subtle way not hearing where the speaker ends and the sub starts or do you want to feel your subs on your chest? Subs that pressurize the room or not so much. There is a reviewer on yt nemo propaganda that does sub reviews. He's on of the better audio reviewers I've run across and he happens to have lots of videos on subs. He is thorough and detailed and is great at articulating sound characteristics in his reviews. He also compares the subs he's reviewing with other ones in it's class. Anyways hope this helps to narrow your search down. Best of luck and happy huntings.