A second vote for the Lightning, even though it's beyond the stated $500. Cheaper subs may be great for HT but to pair with a speaker as good as the Model 1 you'll be severely let down with something in the $500 range. It will be really tough to integrate a lower-end sub and still sound cohesive musically.
Sub for Totem model 1
Hi I have Totem model 1's with muse 100, melos sha 1, mac hard drive into squeeze box2 into ack dac 1.3 would like to get a sub, any recomendations? Can this be done for 500$? Thanks