You might need a heavier Board/baffle. (You might get another board and glue and screw to it, making it twice and thick/heavy. I know you are in the test stage. Those big 15" woofers need good/solid mounting...)
Are you tilting the speaker back, because your horn might need to be mounted higher. They need to be pointed at the ear or at least at ear/listening level....
And I'm not sure how long those take to break in, so give it some time and the sound will change.
V...
Modified: I have been running my Eminence 12FLA's for a long time, but not real loud. I am not sure how many hours I have in them, but, they just got broken in last week. I started playing/using them around six weeks ago..)