For a speaker as quick in transients and dynamics, I suggest one of the Zu subwoofers over the, the Undertone being closest in price to Upscale's blowout price. Zu's subwoofers are the only subs I've heard (including the Tannoy) quick enough to seamlessly match up to fast-transients, responsive speakers like Magnepan, Zu, Quad ESL, and similar.
Phil
Phil