take a 15 inch sub with a high excursion and low fs, put it in a big ported box tuned low, your not going to get a good top end with any sub. Your bottom end is going to be huge, but that was your goal in the firstplace.
I've only played with my ava briefly, but in a 2.1gross box it has a pretty flat response, not too overbearing on the lows, but it does dig deep.