you can trade in your old 350 for a 500/600, that is what i did, traded in my 350 for the 600, i think i got 70% of what i paid for the 350.. so i was only out of pocket 1k$ for the 600.. im sure the difference between the 500 and 600 is not that big, the only real difference, is that the 600 will play lower, due to the cabnet size.. exact same driver/amp in both subs..(i think)