Adding sound dampening will not cure a "tinny" sound. All it will do is remove a lot of money from your wallet in exchange for a lower noise floor.
The 'sound' you're experiencing can be caused by a gritty sounding DAC in your HU, improperly aimed tweeters, and/or a cheap passive x-over. If it's the HU, buy a different HU (old school Eclipse radios sound quite smooth). If it's the tweets doing weird things from either being pointed at you, the glass, whatever - try moving them around a bit. If it's a peaky speaker or tweeter, you'll need an EQ to mellow it out. Good passive x-overs will do this while cheaper ones will not.
These would be the most common culprits anyway.
Lastly, you're speakers are in-phase with each other...right?