Best subwoofer for 2 channel Hi End system


I need your suggest because I want to add a subwoofer to my Hi End system.
I have a pair of Tannoy Edinburgh HE loudspeakers with a Pass Aleph amplifier. Althought the bass are very good I want to add a subwoofer to reproduce very low freq.
Please let me know your opinion and recomendations.
Thanks.
elduende14
I would suggest the green mountain audio hammer. Better yet, two green mountain audio hammers.
I met Roy of Green mountain at the show and he is a hands on kind of guy. I have always had much respect for his work but after meeting him , the respect carries to the man as well.
I just picked up a Martin Logan Depth for my Aeris I. What a great Improvement over the Vel HGS-10.