Assuming a typical 2-8v gain structure, I would have to agree with gains maxed, 5v should more then overdrive the amp with a sine tone recorded at 0db...
There are a couple of other factors which could be considered, but I'm sure we can at least agree extra preamp voltage can't hurt his score.. and for the $50 it might cost for a line driver I'd be more then willing to try one out...
Also, if the OP wants to compete in bass race (with music).. the extra preamp voltage would certainly be of a huge advantage...