I recently got a pair of Elac Varro RS700. They are well engineered, fast and musical subs that go deep. I believe front firing sealed box subs are more accurate.
FWIW: Even though your Luxman has pre out, it is generally recommend to use high level outputs for better integration of subs with speakers. REL, Elac and others offer this option of piggybacking wires off speaker terminals to provide signal to the sub, which the sub then amplifies.
Whatever you choose you can look forward to a much more satisfying hifi experience!

