So you're running a sub off of one, non bridged channel on a two channel amp?
If so, run it at 4ohms for 180W if your VC config allows it - if not, run it at 2ohms and just be careful with the gains.
You can get away with quite a bit of headroom (most I've done is 700W RMS over sub rating), as long as you use your head.
~vR
If it is a 2 channel that does 180x2 at the VC of your subs, just get another sub - those W1 series are cheap, and more cone area is always a good thing.