Subwoofer

+2 for what @grislybutter said.

Integrating a single sub (which is how I started), especially if used in a 5.1 system as well as 2 channel can be challenging. 

For 2 channel I think it's helpful to think of the sub as a low-bass extension of your mains. So get a sub that is designed for that application and place them near each speaker. On the outside and a few inches behind is a good starting point.

It's really not that different from speakers with an integrated/powered sub, but I'd argue better if you get the right combo. Then just experiment with gain and crossover.

FWIW I use  I use two REL T/x9s which I think are very good value for the money but there are several brands that will do the job nicely. Pics in my profile if interested.

The only single sub that I would consider is the Vandersteen Sub 3.

I has an internal graphic equalizer, so you can tune it to the room. Unfortunately, not as inexpensive as other routes, but you get what you pay for.-Especially when you consider Vandy subs like the 2w are still working after 30+ years.

Bob

You can usually make a single sub sound great in one listening position. Not a problem.

Multi sub systems smooth out room nodes so that bass can sound even handed and non booomy in more locations.

That said I’ve EQ’d bookshelves enough that I listen to them more often sans subs.

To have one to three subwoofers work to their integration’l potential at your listening position its imperative to preform the subwoofer crawl to map out your rooms bass mode standing waves (optimum subwoofer positions).

https://www.youtube.com/watch?v=AV3oLLMgS-M

Another excellent option would be to look into ’distributed bass array’s’, Swarm.

A reluctant suggestion would be -6dB sub-bass speaker. Their dramatic frequency roll off may not excite your rooms modes, expanding location possibilities. Hopefully still provide a modicum of lowish frequency reinforcement. Good luck with your search.

Give SVS a call they are very knowledgeable and will walk you through there product line to fit your needs.They also offer a trial period so you can send it back if it doesnt work out.

Good luck

I can assure you the Akg K340 are now optimized and so good i will never go back to the difficult task to tune a room... I dont need because  the three dimensional "out of the head" soundfield of my headphone beat my past speakers or rival them  ...

The K340 is unique headphone the most different designed headphone ever compared to any other... Read about it...

My best to you and your family...

thanks @mahgister I hope you are enjoying the music whichever ways, maybe nearfield listening with speakers is an option too!

thanks @mahgister I hope you are enjoying the music whichever ways, maybe nearfield listening with speakers is an option too!

Tekton 2-10, 4-10 or 6-10…. Whichever suits yer fancy and WAF.

I think grislybutter is right...

I bought a subwoofer years ago because my MIssion Cyrus never go down 45 hertz really...

With an organ music album it is a pity...

But the sub alone dont cure the problem and worsened it because the unbalance it created...I sold it on the spot... :)

I think it takes more than one sub  to balance the system and the room...

 Sadly at first but fortunately at last , my headphone replaced my dedicated room now because i sold my house... My headphone go so low under 30 hertz that i feel the wave with my body and feet by bones resonance and it is like the owgan was in my room... No sub is needed...And my new headphone beat my past speakers in their well tuned room..

I don’t think one subwoofer is a good idea, in general, in addition to speakers with reasonable base. Two can enhance and enrich the sound. One will just muddle it while it will sure add the thump, which I will need for 3 minutes a day.

(and I wish it weren't the case, I tried for almost a year now with different (single) subwoofers)

What’s your budget, and any size limits/restrictions?

The issues with no choice in placement is dealing with room integration. Any subwoofer which is big enough will do IF you have the appropriate measurement and EQ gear.

Personally a huge fan of HSU but I use mine with a miniDSP or the room correction built into my HT receiver. 

i think the dual driver helps with room nodes and if you only use one subwoofy

KEF dual driver subwoofy gets good reviews

https://international.kef.com/products/kc62-subwoofer

I think this SVS is in my future

https://www.svsound.com/products/3000-micro-subwoofer