Subwoofer Recommendations

I am looking for a single subwoofer that will compliment my JBL L100 Classic speakers.  This is my office system and I only want 1 subwoofer (no swarm) to add a little oomph below 50hz.  I will be using it with my McIntosh MA6600 on the second set of variable line outs.  The sub has to accept line in, have a phase control, level control  and adjustable crossover so I can cross it over under 80hz or so.  I will be still running my JBL speakers full range.  Price is up to $1500.00 new or used although keeping it at around a grand would be great.  This is the only way I will hook up the sub and I would like something that does go deep.  I know there are probably hundreds of subwoofer threads but I have never looked at them.  This is just something I came up with today so any help would be appreciated.
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Small footprint which is fine in an office.
Can be placed along the wall
( I ordered one )

Thank you for responding but I don’t think the Kef will work with a couple of 6 inch woofers. Doubtful the bass would go low and deep enough. The JBL speakers have 12 inch woofers, I would think I would need at least an 8 inch woofer as minimum but not sure.
Given your requirements, I would look at Hsu subwoofers.
You should be able to find them used and even new, they are very reasonably priced and work well.
I am using a pair with my Vandy VLR's and find them easy to set up and enjoy.
I think you’re making a mistake going with only one sub with your budget as two subs yields much better bass integration and performance.  For $1500 you can get two SVS SB2000 Pros that go down to 19Hz and include integration electronics, or you can cheap out and get a pair of SB1000s for $950 that’ll still get you down to 24Hz.  Even if you start with one, if I was you I’d eventually get a second.  BTW, SVS offers an extremely generous risk-free trial period that includes shipping, so if two subs aren’t a significant improvement you can always just return one.  Anyway, that’s what I’d do FWIW.  Best of luck. 
I have owned this JL AUDIO D110 and I think it will fit the bill. It does not have a high pass filter if that is important to you. The JBLs will run full range. I’ve owned many subs and I thought this one was outstanding!