Looking for a good small subwoofer...

KEF KC62 - recommended by a composer - studio producer friend to enhance the impact and bass weight of my full range single driver horn loudspeakers: one unit does precisely that. 10" cube, potent, works great.

Pleased to see three supports for KC62.  I have been looking at these a while.  Any more opinions please?

The Kef KF92 and KC62 are two of the best subwoofers available today. They are balanced force designs which means the enclosure will not shake and they have full two way crossovers. With the OP's horn speakers I would want to see two KF 92s. Good for Kef!! 

In this system there is a separate preamp and 40 watt (20/20) EL-34 tube amp currently powering a pair of Kilpsch RP-600M speakers.

Ideally the setup should work like this:
Preamp out (line level) to the sub or subs. 

Stereo line level crossover inside the sub, around, say 100Hz. Lows goes into its internal solid state amplifier, high-pass filtered audio to output jacks which then connect to the tube amp and to the full range speakers.

I'm not sure which of the subs suggested here will accomplish that.

The cost should be around $500 or less so some of the excellent subs suggested here will not fit the bill.
8'or 10" would be more than enough.
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So far these have been suggested:
Thank you all.

 mijostyn: Kef KF92 and KC62

psf4972: KEF KC62

aggregatebuzz: BIC America F12

waytoomuchstuff: Small B&Ws

balooo2: RSL 10 MKII

dbphd: KEF KC62

dwest1023: B&W DB3i

jjss49: rel t5i/x or t7i/x

sunshdw: SVS 3000 Micro

dweller: Polk Audio PSW505

musicaddict: The SVS SB-2000
    
jayctoy: Pioneer, $349
    
tweak1: GoldenEar ForceField 30 - 8" or 40 - 10"
    
fuzztone: SVS micro or KEF KC62

rocray: Rythmik,REL,or SVS

lak: small REL's

grislybutter: Kef, (not a fan of SVS, REL)

hilde45: Rythmik subs

bigshutterbug:  Totem Kin

soix: SVS SB1000 Pro

Here’s one…

https://www.svsound.com/products/sb-1000