sub-woofer recommendation for 2.1 channel


I'm currently using Primare i21 integrated amp and Totem Model 1 Signature speakers.
Recently, I set up the speakers in Joachim's method (http://www.audiophysic.de/aufstellung/beispiel_e.html) and realized that bass is very weak. So I started to think about getting a sub-woofer that I can hook up to the Primare integrated amp.
Do you think it's a good idea to connect a sub-woofer to the 2-channel amp? If so, can you recommend an affordable one that can be a good match with the current system?
Thanks in advance.
henryjudy
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.
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.
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