I'm not doubting you @hickamore but I thought the Definition subs were equipped with dual 12" drivers not 15" as mentioned. If the dual 12" model retailed for over $7k, I'd assume the dual 15" model would cost even more. ($9k?)
At that price you'd have to drive past a lot of proven designs from the likes of REL, JL Audio, and Rythmik. That's probably the main reason you don't see a lot of discussion on the Tannoy sub, not that it's not worthy of discussion.
Best of luck with the integration in your system!