Reddit mentions of ELAC S10 Debut Series 200 Watt Powered Subwoofer by Andrew Jones

In that price range with those size constraints the options are limited....

https://www.amazon.com/Debut-Powered-Subwoofer-Andrew-Jones/dp/B014GSEQ2O/ref=sr_1_7?s=electronics&ie=UTF8&qid=1495463071&sr=1-7&keywords=elac

https://www.amazon.com/Pioneer-SW-8MK2-Designed-100-Watt-Subwoofer/dp/B008NCD2PC/ref=sr_1_7?s=electronics&ie=UTF8&qid=1495463136&sr=1-7&keywords=pioneer+sub

https://www.amazon.com/Dayton-Audio-SUB-800-Powered-Subwoofer/dp/B0063NU30K/ref=sr_1_1?s=electronics&ie=UTF8&qid=1495463193&sr=1-1&keywords=dayton+sub-800

https://www.amazon.com/Klipsch-Reference-R-8SW-Surround-Subwoofer/dp/B01MTGCKCU/ref=sr_1_1?s=electronics&ie=UTF8&qid=1495463226&sr=1-1&keywords=klipsch+8+inch+subwoofer

I think the Dayton may not do your KEF's justice though.

Doesn't have speaker level inputs you will need to use the sub out on a computer sound card or pre-out on an integrated amp.

https://www.amazon.com/Debut-Powered-Subwoofer-Andrew-Jones/dp/B014GSEQ2O

I happened to have this sub I paid $180 for a long time ago, and it actually works well with the Ambeo so far. I never actually tried the Ambeo without it yet. https://smile.amazon.com/gp/product/B014GSEQ2O

When I demoed it in the store, I thought the bass was fairly strong on its own.

I'm curious to what you think it sounds like compared to the playbar.

FYI if you need to do a true reboot of the Ambeo, and don't want to pull the plug, there is a restart option in the Google app. Not the Sennheiser app strangely enough. I had to do it a couple of times while setting things up and it was acting funny.

Good call. I forgot to include the sub. I was looking at a variety of things so never pulled the trigger on one.

Klipsch R-10SW

ELAC S10

Bic Acoustech PL-200 II Subwoofer

If you are looking for a nice easy package check out this. I'm partial to two channel receivers. Harman Kardon 3700 w/ Elac B5 speakers designed by Andrew Jones who is one of the biggest names in high end speakers. Great sound all around here.

https://www.amazon.com/Harman-Kardon-2-Channel-Receiver-Bookshelf/dp/B01M71MF37/ref=sr_1_13?s=electronics&ie=UTF8&qid=1497991941&sr=1-13&keywords=harman+kardon+speakers+receiver

Pair it with matching Woofer from Elac and Andrew Jones.

https://www.amazon.com/Debut-Powered-Subwoofer-Andrew-Jones/dp/B014GSEQ2O/ref=sr_1_2?s=electronics&ie=UTF8&qid=1497992155&sr=1-2&keywords=andrew+jones+elac+subwoofer

$700 total for a kick ass set up for movies and music.

Thoughts on the Elac sub?
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I'm not sure it makes sense to spend that much on the sub. Unless you are the model customer for Beats and/or only watch movies made by the NRA with the intention of having 100% fidelity on the actual deafening sound of a gun, you are better off saving money on the sub.

I have this sub in a fairly large room: https://www.amazon.com/Debut-Powered-Subwoofer-Andrew-Jones/dp/B014GSEQ2O/

It's a great sub (also featured in the buying guide you can find in this sub's sidebar: https://www.reddit.com/r/Zeos/comments/5b1u99/guide_subwoofers/). I would buy it, test it, and move to something louder if and only if you find it's not enough for you. Amazon has free returns anyway, and even if you do decide to return it for a bigger/more expensive sub, you get to avoid buyer's remorse over thinking you got a sub "too good for your speakers".

Its bigger brother (https://www.amazon.com/S12EQ-Powered-Subwoofer-Andrew-AutoEQ/dp/B014GSEUJ8/ref=sr_1_1?s=electronics&ie=UTF8&qid=1522284172&sr=1-1&keywords=elac+S12) is also a widely renowned sub. You could also buy both, try them, and decide which one to return.