Before attempting this, you should be mechanically and electrically proficient. If not, then you should have this done by knowledgeable people. You should possess good quality tools as well. I use the micro-tech screwdriver set from Craftsman with hardened tips as well as a Craftsman electronic plier set. An unmolested medium Craftsman phillips for the drivers/transducer is necessary as well. You don't want to strip out screw heads doing this.
First step is to remove the driver from the cabinet. For the L-300, it is highly recommended that two people perform this operation. The driver is heavy and because of the ten pound magnet at one end, it's like handling half of a 20 lb barbell, no easy feat in the confines of a speaker enclosure. For the L-300, you first remove all the OUTER screws on the removable panel in the back of the speaker cabinet. Do not remove the inner screws, these are for the wood bracing for the motor itself. Pull the back panel off, as the midrange unit will now be supported by the front of the enclosure. The back panel is a very tight fit and you may have to reference the next step and loosen the screws around the horn mouth before the panel will come off. The panel is directional to accomodate the upward slant of the motor (there is a small wooden wedge on the bottom brace to take up the difference between the top of the canted motor and the bottom).