If you want augmentation only, any recent small REL would do nicely (Britannia 3, R-305); if you want to high-pass the Tannoys at a relatively high frequency, say 80Hz to 100Hz, you need a transparent active crossover such as the Hsu Research high-end crossover and a pair of very fast subwoofers such as JL Audio Fathom f112 or ACI Force/Titan XL. There are plus and cons to each solution, of course.
