We have snake springs in our 4 year old sofa and they provide little to now support.  Love the fabric and shape of the sofa, so want to try to make this work.  Can I add coil springs below the snake springs to provide extra support (especially under the seat)?  I also fear there aren't sufficient scross wires installed perpendicular to the snake springs.  I prefer not to cut out a piece of plywood as my solution.