With your amp you must read how to set up the amp and set to gain and other controls.
I would say get one of these http://bazooka.com/productAutoRawWoofersRS.asp?id=165
and build a sealed enclosure for it...1 cu ft as stated by the site.
This sub will be able to handle the power of the amp better than a tube can.
(If I had a 500/1 I would never buy bazooka subs...a 500/1 is able to easily power a JL 12W7...that amp puts out more like 650 watts than 500...underrated...I would get a much beefier sub...I would take advantage of the sweet power of the 500/1...If you want bazooka subs than the 250/1 would be a better match for one bazooka)