Recommended subwoofer pairing with KEF LS50w?

[Ultrasonic]

JL Audio D110
Better than the rest. Better drivers, better amps.

If a sub being small is a primary consideration then I'd consider JL Audio subs like this, but there is a big price premium for working to get decent performance out of smaller boxes and drivers.

 

[tecnogadget]

You will struggle to find any sub with the DSP capability of a 2x4 HD or SHD I think. Whilst the likes of Arendal and SVS offer some DSP capability it is much more limited and harder to optimise. I use a PSA S1510 for music and am glad it doesn't offer any paramtric EQ as I'd basically be paying for functionality that I'd ignore in favour of using my SHD. I live in the UK where Rythmik subs sadly aren't available and so I can't comment from experience, but on reputation I think a Rythmik sub with external DSP sounds like a very good option. Provided you are happy to spend a bit of time learning how to use REW and making measurements.

Exactly, I would not consider the simple 3 PEQ available on SVS or ARENDALan endgame. But they are certainly very handy. In my particular case, I will take advantage of the available PEQ in SVS SB1000 Pro, to tame very raw and specific room modes, smoothing things a little bit without being overly aggressive, this would be Phase 1. Then on top of that built-in PEQ correction, I will run further measurements and FIR filters without Taps restraints through HTPC.

 

[muslhead]

I have used svs, Rythmik, JL audio, DefTech and RSL with my LS50's
My preference was the Rythmiks but my preference would likely change depending upon the room, volume, other equipment and if using EQ as there was not that much difference at the top of the list. The worst by far was the DefTech and one i would never do again.
If budget was at the top if the list of importance, the SVS and RSL would be my go defaults.