Sounds more like you are looking for a sub...

Kinda beating a dead horse here. The 515 is a great MID BASS.

My suggestion? Sell the 515's.

Get a pair of 3154.

The 3154 can be purchased cheaper than the 515 you may come out ahead money wise, and will be a lot happier.

You can improve the lows in a properly tuned box, but if you are looking for 150 down, the 3154 or even a single 8154 is king.