Subwoofer opinion badly needed!

Hi. I'm looking for a sub to be used primarily for MUSIC (say, 98% of the time), that costs under $500, and will be used in a small/mid-sized room (approx. 18x12') with the following: Harmon Kardon AVR20II receiver Infinity RS225 (reference series speakers) I've had the following recommended but am still uncertain and would really like some 'sound' opinions. Keep in mind the main use is for music. Thanks!