Sir the best suggestion I can give you is to use a dedicated SUB like Alpine Type E, it will be better then any 6 x 9 on the planet.
Alpine Electronics of America, Inc.
Else these Fusion can get really loud and clear without breaking the bank.
PP-FR693
Also, what amp are you using?
The 6975 are definitely much better but they also cost a fortune and they cannot compete with 2 pairs of 6993 in output.
Regards