Transmission line subwoofers to go along with Rogers LS-3/5A speakers that I used for the top end. They were 5 ft tall by 24x28", using KEF 6x9 flat woofers in folded backside termination. Sold them.
Nice 100W/channel design published by Marshall Leach from GA Tech back in the '70's. I love the sound, and have been using it for over 35 years.