I'm suprised your 15 is bottoming, I push mine hard and have never bottomed it. My PR would bottom when pushed but once I upgraded to the big bump PR, I haven't had that problem.
You will need to consider the internal volume of the cabinet for your choice of drivers, some will work better than others. You shoul run Simulations in software to confirm the performance.
There is a TC Sounds 15 on ebay now you could look at.