Best subwoofer for 2 channel Hi End system

I use a Velodyne HGS-15 with a pair of bi-wired KEF 104/2s. A Proceed processor sends everything below 80 Hz to the sub, which results in even more open and transparent sound, even though the KEFs are capable down into the 20 Hz range. Freeing stereo speakers of low frequency duty seems beneficial. (I use a Proceed Amp-3, and each channel can deliver 250 watts into the 4 ohm load of the efficient KEFs.) The KEF to Velodyne transition is seamless. I plan to add another HGS-15 on general principle, even though LFE is smooth and strong throughout room with a single HGS-15.