An alternative to TLS Guy's suggestion for 4 inch mid-range drivers would also be 5¼ inch mid-bass transducers.\n\nI have been using a pair of Peerless 830991drivers in each front speaker cabinet. They have a 5¼ inch diameter cone and deliver a rather smooth frequency response to at least 5 KHz.\nIn my three way enclosures, they operate between 200 Hz and 2,900 Hz without any null within the set range of operation. They are amazing mid-range drivers. Parts-Express are out of stock for them at least until December, but Solen.ca has 51 of them in stock. I don't hesitate to recommend them for any serious DIY speaker builder.