Most rap extends to 35hz, below your tune the amp takes care of lessening distortion with the subsonic filter, just play with the filter with the amp at full tilt on music you consider to get low, the tune might be 35-40, but your sub might still get some output below that better than another, I'd tune to 35hz. I was tuned around 36-37hz last, and even with the filters at 50hz/50hz on my 1500W amp there wasn't too much of a difference, and it had much better SQ on the roll off. If anything, make sure you build the box right or the amp and sub won't even matter. You don't want a bandpass box they sound like , if anything, get 2 more of the subs you have now and seal them in 1 cuft each, should sound tight, loud, and get very low depending on your subs!

 

So basically tuning to 35 HZ is safe, will produce good deep base, and my higher frequencies will still be pretty decent? I figure spending another 40$ on a sheet of MDF is better than spending another 500-750 on another amp and different subs just to get it lower and deeper.