stereo system advice from Experts

i would suggest a single 15" sub dual 2 ohm so you can run your sub channel @1 ohm and  a vented box

I don't think your amp will have any problem powering a 15" sub - those aren't minimum wattages you're likely reading.  15"+ subs are usually built to be ABLE to handle more power than 10's etc b/c that's typically how they're used, but in terms or power requirements there's not much difference.  Look at the specs & check the number for "sensitivity."  Compare to what you're using now for a reference.  This is the amount of sound output with 1 watt of power measured from 1m away.  As long as you get something comparable or better you're in good shape.  Expect numbers to be around 85-90db, the higher the better. 

A ported/bandpass box will be more sensitive at certain frequencies, meaning louder with same power than a sealed box, but less balanced, less controlled & more colored.