Is there anywhere else in your area? Best Buy is pretty much as bad as it gets when it comes to installations. They will get the speakers in there, but won't do a single thing else.
Here's a pic of me installing my set. You can kind of make out the deadener that was already in place before i started. I actually consider the seals that are surrounding the speaker, and close the gap in between the speaker and the plastic door panel, to be more critical when it comes to getting the most out of the mid. Best buy will NEVER do this. Probably won't even know what you're talking about if you ask them to, lol.