pass x-350 vs x-600

Has anyone made the jump from the 350 to the 600 monos? Were there any audible improvements? Ive heard that the circuit design is different but Im not sure if thats true. My speakers are 91db efficient Dunlavy's and im not sure if the extra power would make a difference. Thanks.
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.

My room is 13x17 also but I have the speakers against the long wall which leaves me 9-10 ft. from the speakers and my head 6" from the back wall. No dedicated line but am using a audience cord, a x-2.5 pre and the emc-1 cd player. I dont mind spending the cash if the result is worthwhile.
No doubt, x-600 wins x-350 in every aspect. Especially, the improvement on bass control and sound stage stands out quite a bit. I upgraded to X-600 from Aleph 2. However, I have compared X-600 and X-350 before.