I made the jump, and found it worthwhile. There was an improvement in the whole spectrum, but particularly in the lower frequencies. I am a believer that for large/inefficent speakers, the more power the better. I guess whether it is cost-effective for you depends on a lot of other things including size of room, other equipment, price of selling the x350 and acquiring the x600, etc. You might consider other improvements before making such a huge jump, such as a dedicated line, suitable power cord, etc.
Rob
Rob