sub-woofer recommendation for 2.1 channel

Look into this sub from RSL.

http://rslspeakers.com/RSL_Speedwoofer_10.html

It is the sub I bought for my maggie 1.7's and the only one
I could get to blend with them. Now I sold the maggies and
have Martin Logan CLS. This sub should blend perfectly with your speakers, and if for some reason you don't like it, (I doubt that will happen) you can send it back for a full refund, they even pay return shipping! Also, it includes a
remote to let you adjust X-Over & volume from your listening
position. Another great feature at this price point!!

@Johnnyb53, Thanks for your answer!
It was very helpful.

A long time ago, I had the Primare A20 MKII (your amp's ancestor) with Dynaudio 52 monitors.

I used the REL Strata III sub via high level outputs; it blended very well.

I don't think you need to spend big bucks to get the benefit you're looking for. Certainly closer to $500 than $1000, put it that way. If I was buying again, I would think about a kit like this:

http://www.madisoundspeakerstore.com/subwoofer-speaker-kits/sb-acoustics-12-powered-subwoofer-kit/

03-15-12: Henryjudy
Thanks for your recommendations on the Martin Logan subwoofer.
However, I'm not sure how I can connect the ML subwoofer to the amp.

Please take a look at the pictures of the speaker and amp.

http://www.audiovideostore.com/images/MartinLogan_Dynamo500_front_02.jpg

http://big54.ru/published/publicdata/A229541/attachments/SC/products_pictures/Primare-I21%283%29_enl.jpg

You're all set for a good connection. Your Primare has a pair of pre-outs. That's preamp outputs tied into the selection and volume settings on the front panel. Connect a long pair of RCA plug interconnects from the Primare's PRE OUT jacks to the Martin Logan Line Level inputs, set the toggle switch to Crossover Variable (L&R In) so you can adjust crossover point and level settings and you're there. You'll have to experiment with placement and flipping the Phase switch for best integration.

I use a Velodyne sub with the high level speaker inputs. All you have to do is run another set of speaker leads from your speaker outs on the amp to the sub and you can use the crossover built into the sub to get it where you want. I have it set low and volume just enough to hear.
-Velodyne DLS
I think they offer it from either an 8/10" up to the 15"
Seems to work fine for the price. Hope this helps

Thanks for your recommendations on the Martin Logan subwoofer.
However, I'm not sure how I can connect the ML subwoofer to the amp.

Please take a look at the pictures of the speaker and amp.

http://www.audiovideostore.com/images/MartinLogan_Dynamo500_front_02.jpg

http://big54.ru/published/publicdata/A229541/attachments/SC/products_pictures/Primare-I21%283%29_enl.jpg

ML sub is quite good with user-friendly dials (on the Descent which i've auditioned with CLX). I like Velodynes because they are super-easy to finetune which is a very important part of integrating a sub into a system. DD10, 12 and DD18 are all excellent. I run the DD18 at the moment, and used to have an older model from an earlier line.

I think Totem makes one. I am experimenting with a Marton Logan sub-which is actually working out pretty well. Its fast and if I dont push it hard, rounds out the bottom end very nicely.