Audio Systems: car speakers, polyfill, car speakers

Question
I have a 67 Vette which I want to improve the sound system. I have started to build a speaker cabinet for Focal 260 V3 speakers. When I connect to audio source I am getting reverb(echoing) and low mids. Bass is nice. Any ideas on how to improve-baffling, materials,etc. Thanks in advance GGW

Answer
well the cabinet needs to be heavy and thick so that it doesn't resonate.  it also has to be deep enough so that the waves off the rear of the cone don't bounce back through the cone.  that will cause very spikey peaky performance.  stuff it very full (packed down if possible) with polyfill and see if that helps.
start there and let me know.