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