With two cabinets, both should be rated at 8 ohms. When connecting an extension 8 ohm cabinet, use the ext speaker jack, NOT the ext amp jack. Ext amp has a voltage divider on it the knocks down the level to around 20%. If you haven’t added a second parallel jack to replace the 4-pin connector, connect the main cabinet to the EXT SPEAKER jack, leave the 4-pin disconnected. Use the daisy chain on the back of your main speaker cabinet to connect the second cabinet.